core/fmt: Second attempt at improving number formatting

[BunterSchatten]

This pull request tries to improve the number formatting of the core/fmt package to do what the docs say it should.

Notable changes are:

• default padding is now " " instead of "0" (fixes [fmt] left padding of numbers should default to space instead of 0 #3112)
• right padding is now always done with spaces, to keep the formatted number correct
• the space flag now adds a space, when there is no sign
• the plus flag now also adds a space before NaN
• the hash flag now correctly adds a base prefix (eg. "0x") to number and removes it from pointers
• the base prefix is now counted towards the total width

I didn't use the suggestion from #3115 (comment), because I didn't want to change the formatting syntax.
This is my second attempt at improving number formatting, I'm closing the first one (#6148), because I found a few other problems afterwards that needed more extensive changes.

Here are a few formatting examples that are changed by this pull request.

// format                        old           new
// ------------------------------------------------

fmt.printf("%.0i",  0)         //  ""       ->   "0"

fmt.printf("% i",  1)          //  "1"      ->   " 1"
fmt.printf("% i", -1)          //  "-1"     ->   "-1"

fmt.printf("%4i",  1)          //  "0001"   ->   "   1"
fmt.printf("%4i", -1)          //  "-001"   ->   "  -1"

fmt.printf("% 4i",  1234)      //  "1234"   ->   " 1234"
fmt.printf("% 4i", -1234)      //  "-1234"  ->   "-1234"

fmt.printf("%-4i",  1)         //  "1000"   ->   "1   "
fmt.printf("%-4i", -1)         //  "-100"   ->   "-1  "

fmt.printf("%- 4i",  1)        //  "1   "   ->   " 1  "
fmt.printf("%- 4i", -1)        //  "-1  "   ->   "-1  "

fmt.printf("%+4i",  1)         //  "+001"   ->   "  +1"
fmt.printf("%+4i", -1)         //  "-001"   ->   "  -1"

fmt.printf("%+0 4i",  1)       //  "  +1"   ->   "+001"
fmt.printf("%+0 4i", -1)       //  "  -1"   ->   "-001"

fmt.printf("% 04i",  1)        //  "   1"   ->   " 001"
fmt.printf("% 04i", -1)        //  "  -1"   ->   "-001"

fmt.printf("%-04i",  1)        //  "1000"   ->   "1   "
fmt.printf("%-04i", -1)        //  "-100"   ->   "-1  "

fmt.printf("%- 04i",  1)       //  "1   "   ->   " 1  "
fmt.printf("%- 04i", -1)       //  "-1  "   ->   "-1  "

fmt.printf("%#04x",  10)       //  "000a"   ->   "0x0a"
fmt.printf("%#04x", -10)       //  "-00a"   ->   "-0xa"

fmt.printf("%#+04x",  10)      //  "+00a"   ->   "+0xa"
fmt.printf("%#+04x", -10)      //  "-00a"   ->   "-0xa"

fmt.printf("%#-04x",  10)      //  "0xa0"   ->   "0xa "
fmt.printf("%#-04x", -10)      //  "-0xa"   ->   "-0xa"

fmt.printf("%#-4x",  10)       //  "0xa0"   ->   "0xa "
fmt.printf("%#-4x", -10)       //  "-0xa"   ->   "-0xa"

fmt.printf("%# 04x",  10)      //  "   a"   ->   " 0xa"
fmt.printf("%# 04x", -10)      //  "  -a"   ->   "-0xa"

fmt.printf("%# +04x",  10)     //  "  +a"   ->   "+0xa"
fmt.printf("%# +04x", -10)     //  "  -a"   ->   "-0xa"

fmt.printf("%# -04x",  10)     //  "0xa "   ->   " 0xa"
fmt.printf("%# -04x", -10)     //  "-0xa"   ->   "-0xa"

fmt.printf("%+f",  NaN)        //  "NaN"    ->   " NaN"

fmt.printf("% f",  3.14)       //  "3.140"  ->   " 3.140"
fmt.printf("% f",  NaN)        //  "NaN"    ->   " NaN"

fmt.printf("%7.3f",  3.14)     //  "003.140"  ->   "  3.140"
fmt.printf("%7.3f", -3.14)     //  "-03.140"  ->   " -3.140"
fmt.printf("%7.3f", +Inf)      //  "+000Inf"  ->   "   +Inf"
fmt.printf("%7.3f", -Inf)      //  "-000Inf"  ->   "   -Inf"
fmt.printf("%7.3f",  NaN)      //  "0000NaN"  ->   "    NaN"

fmt.printf("%07.3f", +Inf)     //  "+000Inf"  ->   "   +Inf"
fmt.printf("%07.3f", -Inf)     //  "-000Inf"  ->   "   -Inf"
fmt.printf("%07.3f",  NaN)     //  "0000NaN"  ->   "    NaN"

fmt.printf("%-7.3f",  3.14)    //  "3.14000"  ->   "3.140  "
fmt.printf("%-7.3f", -3.14)    //  "-3.1400"  ->   "-3.140 "
fmt.printf("%-7.3f", +Inf)     //  "+Inf000"  ->   "+Inf   "
fmt.printf("%-7.3f", -Inf)     //  "-Inf000"  ->   "-Inf   "
fmt.printf("%-7.3f",  NaN)     //  "NaN0000"  ->   "NaN    "

fmt.printf("%- 7.3f",  3.14)   //  "3.140  "  ->   " 3.140 "
fmt.printf("%- 7.3f",  NaN)    //  "NaN    "  ->   " NaN   "

fmt.printf("%-07.3f",  3.14)   //  "3.14000"  ->   "3.140  "
fmt.printf("%-07.3f", -3.14)   //  "-3.1400"  ->   "-3.140 "
fmt.printf("%-07.3f", +Inf)    //  "+Inf000"  ->   "+Inf   "
fmt.printf("%-07.3f", -Inf)    //  "-Inf000"  ->   "-Inf   "
fmt.printf("%-07.3f",  NaN)    //  "NaN0000"  ->   "NaN    "

fmt.printf("%- 07.3f",  3.14)  //  "3.140  "  ->   " 3.140 "
fmt.printf("%- 07.3f",  NaN)   //  "NaN    "  ->   " NaN   "