Add internal block type for use in the compiler. IncrementalBuilder only maintains two states

[TIHan]

This is mostly a refactoring PR. It does add the block type for internal compiler use; it is basically ImmutableArray<'T> but with F# idiomatic collection functions.

Incremental Builder changes:

• Added IncrementalBuilderInitialState to group the initial state when creating an IncrementalBuilder internally.
• Lifted a lot of local functions in IncrementalBuilder to be top-level.
• IncrementalBuilder now manages only two states: IncrementalBuilderInitialState and IncrementalBuilderState.

[NinoFloris]

I did a review of the block api, I hope that's alright.

Also, sometimes there are reasons to get the underlying array cheaply. I'm using this, posted below, in our codebase here and there. Though going fully unsafe you could do Unsafe.As<Block<'T>, 'T[]> as it's the only field in the struct. I just prefer my code to die a little nicer than with a segfault in the very unlikely case ImmutableArray changes implementations 😅.

letunsafeGetArray(value:ImmutableArray<'T>)=match MemoryMarshal.TryGetArray(value.AsMemory())with|false,_-> failwith "Expected ImmutableArray to wrap a managed array but it didn't."|true, seg -> seg.Array

[TIHan]

@NinoFloris of course! Thank you for reviewing :)

[TIHan]

@NinoFloris This should be a lot better. :) I didn't realize how naïve my original impl was.

[NinoFloris]

No worries, I picked up a few things since I introduced ImmutableArray into our codebase a few years back. Besides, anything that helps compiler perf is welcome :)

[NinoFloris]

If you don't know the length it doesn't matter too much if you do ToImmutable. You'll do a copy by changing capacity anyway.

[TIHan]

Changing the capacity will only copy if the count is different.

[NinoFloris]

Fair enough :)

[NinoFloris]

Missing capacity <- count or ToImmutable here, as capacity could be anything.

[TIHan]

I think this is fine. acc is the total length of the final array.

[NinoFloris]

Ah I completely missed the the length summing loop, yep it's fine!

[dsyme]

Test for empty initially