Refactoring: splitting a chunk from CheckExpressions

[psfinaki]

[dsyme]

@psfinaki Basically this looks good, though I'd like to do some more on it next week to reap more benefit from the changes.

I pushed some adjustments

• Rename CheckTypes.fs/fsi to CheckBasics.fs/fsi. We will use "CheckTypes.fs/fsi" for TcType and friends.

• I moved one thing back to CheckExpressions.fs since it wasn't specifically needed in CheckBasics.fs (the purpose of CheckBasics should be to establish TcEnv and TcFileState and nothing else).

• I'd imagine we will move most of the helpers in CheckExpressions.fs which only relate to TcEnv and TcFileState and the other such types into CheckBasics.fs. I pushed one example of this - often this will involve making things into members according to the types they operate on. This is part of the benefit we can gain by this - more of the logic of CheckExpressions.fs will migrate out and be organized differently.

• I guess we may eventually split CheckBasics itself into TcEnv.fs and TcFileState.fs though no need to do that for now.

We can merge this for now I think and get it in the bank, then iterate next week.

[dsyme]

Just to note that, once we iterate, this lays the foundation for further splitting, e.g.

• CheckAttributes.fs/fsi for TcAttribute* and friends
• CheckTypes.fs/fsi for TcType*
• GeneralizationHelpers.fs/fsi (helpers related to generalization)
• EliminateInitializationGraphs.fs/fsi (EliminateInitializationGraphs and friends)
• CheckIndexAndSlice.fs/fsi (TcIndexerThen* - quite a large chunk)
• CheckObjectExpressions.fs/fsi (TcObJExpr* - quite a large chunk)
• CheckBindings.fs/fsi (normalizing bindings, TcLet and incremental generalization of groups of recursive bindings)
• CheckStrings.fs/fsi (checking format strings, interpolated strings)

[psfinaki]

@dsyme thanks for the review and the adjustments. Yeah here we already have quite a lot of changes, so merging this one - and hope we'll address your points above in the near future :)