* initial support for span, readonly refs, byref-like structs

* fix proto build

* make proto work with previous FSharp.Core

* make proto work with previous FSharp.Core

* update baselines

* integrate code cleanup

* integrate code cleanup

* integrate code cleanup

* integrate code cleanup

* fix build

* fix build

* implicit deref of byref returns

* add tests for Memory, ReadOnlySpan and ReadOnlyMemory

* fix tests

* simplify diff

* simplify diff

* remove duplicate error messages

* fix build

* test updates

* fix build

* fix build

* update baselines

* fix uses of NativePtr.toByRef

* switch to inference using byref pointer capabilities

* fix proto build

* update baselines, byref extension methods

* fix test errors

* emit in,out,modreq attributes correctly

* update tests

* fix build

* fix build

* fix tests

* fix tests

* get it right silly boy

* fix test

* minor cleanup

* add more tests

* clarify overloading behaviour + test case

* fix build break

* fix build of tests

* update tests

* add more tests

* byref fixes

* updates for subsumption calls, error message, assign-to-return-byref

* test updates, implicit deref on byref return for normal functions

* update baseline

* improve debug formatting, better error message on implicit deref, improve error messages

* add more tests for recursive functions

* update baselines

* fix baselines

* updates for new test cases

* updates for new test cases

* test updates and byref-to-byreflike

* deal with 'M() <- expr'

* restrict addresses of immutable top-level things

* fix IsByRefLike on struct

* update tests

* fix test

* fix test

* improve check for no-return-of-struct-field-addresses

* fix test case