Local functions update: Use detection, expression trees, inferred multiple returns

[khyperia]

Things in this PR:

• The type BoundBlock now contains an ImmutableArray<LocalFunctionSymbol> alongside its ImmutableArray<LocalSymbol>. This generates lots of noise in this PR, as a lot of construction sites had to be updated. Note that this was planned from the start, in the original local functions discussion BoundBlock having a list was mentioned, but I just never got around to adding it (as it was never needed).
• Data flow analysis now reports unused local functions. This is the reason why BoundBlock suddenly needs a symbol list only now, this far in.
• References to local functions are now disallowed in expression trees, which may or may not change in the future (Previously they were generated as a reference to a mangled method name, which seemed wrong). Added a new error for this.
• Multiple return statements in an inferred return type local function must all have exactly the same type. This may change later, but for this prototype it's an acceptable restriction. Added a new error for this.

FYI @jaredpar @VSadov @agocke Theoretically these PRs will get smaller and more frequent compared to the first three, but this one is rather noisy due to the BoundBlock change.

[jaredpar]

Looks like this logic is the same as the above function. Move into a helper?

[khyperia]

@dotnet-bot test this please

[khyperia]

@dotnet-bot test this please