remove unnecessary allocations in float and int serialization via SerJson and SerRon

[tadeohepperle]

It is quite wasteful to allocate a new string for every integer or float encountered during serialization to json (or Ron) via e.g. format!("{self:?}").

I replaced all relevant instances by write!(s.out, "{self:?}"), which formats into the output string directly without any transient allocation. If there were any benchmarks I am pretty sure we'd see performance improvements.

[not-fl3]

Whats up with this one? Is this removal intentional?

[tadeohepperle]

Oh, clippy told me that the pattern is unreachable because it is covered in an earlier branch, so I just removed it:

warning: unreachable pattern
   --> src/toml.rs:15:11
    |
 15 |         | '\u{2D}'
    |           ^^^^^^^^ no value can reach this
...
590 |             '-' => {
    |             --- matches all the relevant values
...
601 |             ident_chars!() => return self.parse_ident(i, num),
    |             -------------- in this macro invocation

[tadeohepperle]

@not-fl3 Would it be possible for you to merge this MR already?