[C#] Optimize memory allocations when parsing numbers in .NET5 or greater

[TrayanZapryanov]

Do not use StringBuilder when loading and parsing numbers by using char buffer.
If target is NET5 or greater stackalloc can be used, but due to compatibility reasons we cannot use some Span<> methods.

[jskeet]

Again, I'm not going to have time to get to this any time soon.

[jskeet]

This will conflict with #15794. Do you want me to go ahead getting a final review for that and getting that merged, or is this your preferred approach now?

[TrayanZapryanov]

Again, I'm not going to have time to get to this any time soon.

No worries. Do it once you have time.

[TrayanZapryanov]

This will conflict with #15794. Do you want me to go ahead getting a final review for that and getting that merged, or is this your preferred approach now?

Please review and merge the other one first. This one will be adapted after that.

[TrayanZapryanov]

Same benchmark as here

Main
// * Summary *

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3235/23H2/2023Update/SunValley3)
11th Gen Intel Core i9-11900K 3.50GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK 8.0.200
[Host] : .NET 8.0.2 (8.0.224.6711), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
DefaultJob : .NET 8.0.2 (8.0.224.6711), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI

Job=DefaultJob Error=3.84 ns StdDev=3.21 ns

Method	Mean	Allocated
Small	281.1 ns	1.01 KB
Big	256,221.4 ns	642.7 KB

PR
Optimize number

Method	Mean	Allocated
Small	264.0 ns	936 B
Big	260,213.7 ns	564016 B

[jskeet]

Please could update this in light of the current HEAD? I can then try to take a look next week.