[generator] Remove global state around code generation

[jpobst]

A batch of commits that make generating the type .cs files thread-safe, and enables writing them in parallel. On my 6 core machine, this reduces time taken generating the types in Mono.Android.dll from ~31s to ~5.2s.

[jonpryor]

Overall, I don't think that this is a good idea/is (currently) doomed to fail, for one reason:

Mono.Cecil is not thread safe.

For all binding projects except xamarin-android/src/Mono.Android, Cecil will be used (to resolve types from Mono.Android.dll); see also e.g. ManagedClassGen, ManagedCtor, ManagedField...

So long as Cecil is not thread safe, there's no way to make anything which depends on Cecil thread-safe.

See also:

• [JavaCallableWrappers] Parallel.ForEach per assembly #365
• [jcw-gen] DON'T generate Java Callable Wrappers in parallel #43

I can imagine only two ways to make this work in principal:

1. Migrate generator away from Cecil toward System.Reflection.Metadata, which itself is not thread safe, but it's (1) faster than Cecil for many operations, and (2) easier to reason about the shared state so that you can avoid sharing it; or

2. Figure out how to use Cecil in a thread-safe manner from multiple threads.

One idea tossed around for (2) was to have each thread separately load every assembly, e.g. instead of loading Mono.Cecil.dll once, we'd load it once per thread -- along with all other referenced assemblies! -- and, if we're really careful, things would work.

(1) is probably too much work, and (2) is only an idea, with no confirmation that it could work.

[jpobst]

Fair enough, I think I can make it thread-safe with some other cleanup I want to do.

I want to have a ManagedAdapter and an XmlAdapter that both fill in POCO objects like Method, rather than having ManagedMethod and XmlMethod that hold on to Cecil/Xml references for the whole process. That way the Cecil/Xml loading step will fill in all the POCOs and then we won't need to hold on to the sources any more.

I will look further into that.

[jpobst]

For this PR, I would like to keep all the work around removing shared state, and I have reverted the bit about generating types in parallel. We can possibly revisit that in a future PR.

[jonpryor]

Relatedly: the .OrderBy() is particularly useful now; see also: https://github.com/xamarin/java.interop/pull/442/files#r303698944