Fixes #41457.

* MethodSymbol and FieldSymbol equality fixes:
 - Defer the equality of method/field symbol to the substituted symbol when compared
 - Make the hashcode of substituted method/field defer to the original definition

Rename from IncludeNonNullableReferenceTypeModifier to IncludeNotNullableReferenceTypeModifier (#41332)

This changes the C# compiler so that it multi-targets between
`netstandard2.0` and `netcoreapp3.1`.

Closes #41304

Fixes #40629

See #41273

Fixes #41207.

Fixes #40768.

Moves an ArrayBuilder for DeclaredEnvironments to a single environment.
This means that a scope only declares a single closure environment for its
captured variables.

I left space for us to declare multiple closure environments
in a single scope, but that seems like over-engineering at the
moment and it makes a lot of code more complicated. We can
always add it back if it seems like it would be worth it

Capturing lambdas should be avoided in core compiler code
as they generate excess allocations

Fixes #41054

* Insert correct `WithNullableContext` binder into the binder chain to ensure speculative models return correct information based on speuclation location.

* Refactor Nullable context options to rely on an enum instead of two bools.
Break out the BuckStopsHereBinder's code to allow sharing.
Fix binder ordering issue for TryGetSpeculativeSemanticModelForMethodBody.

* Make NullableContextState.State a byte for space.