wip: ILC & R2R UTF-8 name mangling.

[PaulusParssinen]

wip: Previewing diffs

todo: desc
todo: r2r correctness

Background

dotnet/corert/issues/2178
dotnet/corert/pull/2200

todo: write motivation

This PR

• Introduce ref struct Utf8StringBuilder
  • To mirror already familiar internal ValueStringBuilder but UTF-8.
  • Pooling now happens in the Utf8StringBuilder using the ArrayPool<byte>.Shared
    • Previously the builder instance was pooled (and consequently the underlying per-thread buffer, which did very well). There's pros and cons on both approaches as always but the control over the initialBuffer allows us to avoid allocating or renting altogether
    • Previously only the extension method GetName(this ISymbolNode ..) had pooling logic, but now we can fallback to pool anywhere where we build mangled symbol name strings.
• Avoid intermediate string allocations
  • For the existing a GetMangled[Type/Method/Field/String]Name methods,
    introduce AppendMangled[Type/Method/Field/String]Name overload which can be used to append directly to the existing builder (very common operation)
    • This removes a lot of "back-and-forth transcoding between UTF-8 <-> UTF-16 due to implicit string -> Utf8String cast and its overhead.

Problems and questions

• This PR was inteded to keep as unintrusive as possible for easier review but that turned out to be very hard due to viral nature of symbol names. In my opinion, it was more straightforward to go all in with the Utf8String so this PR flows it through all the way to the ObjectWriter logic. Choosing a cut off point for the Utf8String elsewhere would render null the transcoding/allocation savings.
• pre-existing inconsistent name sanitization. Fix now? Still relevant as this was for primarily for the Cpp backend?
  • Name Mangling Robustness for Parameterized Types dotnet/corert#2175

Potential future improvements

• If we get custom UTF-8 string interpolated handlers, they'll allow tidying the mangling logic.
• IPrefixMangled[Method/Type] could be optimized to append mangled names

Results

dotnet new webapiaot

Stage 2 Goldilocks (TodosApi)

• -100MB of GC heap allocations removed?
  

Compile ILC with ILC

• todo:

Bench with using the NAOT compiled ILC

• todo: how to measure NAOT GC perf