IlxGen parallelization

[T-Gro]

This is right now implemented as an opt-in feature that allows parallel run of generating the code of method bodies, later in the IlxGen process.
It is not a full parallelization, since anything besides method body generation still happens eagerly and sequentially.

This can be enabled either by:
--test:ParallelIlxGen command line switch to fsc.exe
Environment variable Environment.GetEnvironmentVariable("FSHARP_EXPERIMENTAL_FEATURES") |> isNotNull

The environment variable option is preferred to get dogfooding as much as we can.

Current timing for ComponentTests project:

Old, sequential approach:

Parallel approach (most of the code is still sequential though. That is why the impact is not that big):

When implementing this, AssemblyCodeGen had to be made thread safe. While at it, locking has been reduced for certain common global functions.

[KevinRansom]

This looks good. Perhaps have a think about the oversized collections. And also whether there is a good heuristic for sizing collections.
Thanks

Kevin

[T-Gro]

This looks good. Perhaps have a think about the oversized collections. And also whether there is a good heuristic for sizing collections. Thanks

Kevin

Will address this right after this PR.