Monomorphisation: add commutation with `let`

Motivation and Context

It was reported by Heitor that tools/debugger/ligo-debugger/test/contracts/tezos-types.mligo doesn't work with compile contract:

$ ligo compile contract tools/debugger/ligo-debugger/test/contracts/tezos-types.mligo
File "tools/debugger/ligo-debugger/test/contracts/tezos-types.mligo", line 1, characters 34-59:
  1 | let empty () : 42 sapling_state = Tezos.sapling_empty_state
  2 | 

Cannot monomorphise the expression.

There were some changes on how let foo () = ... compiles (expansion, pattern matching).

Description

In this MR we extend our monomorphisation pass with an internal pass that commutes (let x = t in u)@{T} into let x = t in (u@{T}).

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Checklist:

• If a new syntax has been introduced, put a message on slack ligo-lsp
• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).