feat: Tighten specifications on Strings standard library (#4868)

### Description
As follow-up from the initial review, this adds a bodiless
`CharsConsistent` lemma to the abstract
`Std.Strings.ParametricConversion` module that ensures `chars` and
`charsToDigits` are consistent, which is necessary but not sufficient to
prove that `forall n :: ToInt(OfInt(n)) == n`.

Also makes some documentation and naming improvements, adding an
`IsDigitChar` predicate for better readability than `c in charsToDigit`
(which is slightly different from `c in chars`) and merges the two
confusingly-named `ToNumberStr` and `OfNumberStr` predicate into one
`IsNumberStr` (with the definition of the former, weaker predicate).

### How has this been tested?
Existing tests + verification.

Author: robin-aws

Source/DafnyStandardLibraries/binaries/DafnyStandardLibraries-cs.doo

@@ -22,4 +22,8 @@ module {:disableNonlinearArithmetic} Std.JSON.ByteStrConversion refines Strings.
                          '4' as byte := 4, '5' as byte := 5, '6' as byte := 6, '7' as byte := 7,
                          '8' as byte := 8, '9' as byte := 9
                        ]
+
+  lemma CharsConsistent()
+    ensures forall c <- chars :: c in charToDigit && chars[charToDigit[c]] == c
+  {}
 }

Source/DafnyStandardLibraries/binaries/DafnyStandardLibraries-go.doo

@@ -50,6 +50,10 @@ module Std.JSON.Deserializer {
         'a' as uint16 := 0xA, 'b' as uint16 := 0xB, 'c' as uint16 := 0xC, 'd' as uint16 := 0xD, 'e' as uint16 := 0xE, 'f' as uint16 := 0xF,
         'A' as uint16 := 0xA, 'B' as uint16 := 0xB, 'C' as uint16 := 0xC, 'D' as uint16 := 0xD, 'E' as uint16 := 0xE, 'F' as uint16 := 0xF
       ]
+
+    // The size of the map makes this impractical to verify easily.
+    lemma {:axiom} CharsConsistent()
+      ensures forall c <- chars :: c in charToDigit && chars[charToDigit[c]] == c
   }
 
   const HEX_TABLE_16 := Uint16StrConversion.charToDigit

Source/DafnyStandardLibraries/binaries/DafnyStandardLibraries-java.doo

@@ -74,7 +74,7 @@ module Std.JSON.Spec {
       if i == 0 {
       } else {
         var isHexDigit := c => c in Strings.HexConversion.HEX_DIGITS;
-        assert Strings.HexConversion.OfNumberStr(s, '-');
+        assert Strings.HexConversion.IsNumberStr(s, '-');
         assert isHexDigit(s[i]);
       }
     }