Remove IAssemblyName (and various fusion remnants)

[elinor-fung]

Remove a bunch of remnants of fusion from the binder. Definitely lots more that this change doesn't deal with (e.g. the existence of the ICLRPrivBinder interface - it is internal only and our usage of if it already doesn't adhere to COM semantics).

• Replace IAssemblyName with a simple struct (AssemblyNameData) for passing information to binder instances. (If we get rid of the interface per above, we can remove the intermediary struct as well - but it is at least simpler and less costly than IAssemblyName.)
• Delete a bunch of files / functions that were just for dealing with IAssemblyName
• Move some types from deleted files to bindertypes.hpp: PEKIND from peinformation.h, AssemblyContentType and ASM_DISPLAY_FLAGS from fusion.h

This avoids re-parsing of the display name and some data copying, so it actually improves perf some too. Relative to the cost of actually loading an assembly, it is small / lost in the noise, so I tried narrowing down a measurement to the cost of just going through bind resolution by using custom ALCs and calling LoadFromAssemblyName for an already loaded assembly.

Stopwatch timings for 150k iterations using my local build:

	Before - 150k iterations (s)	After - 150k iterations (s)
Returned by ALC.Load	1.8	0.9
Fall back to default ALC	1.9	1.0

Test code

Assembly asm = Assembly.GetExecutingAssembly();
AssemblyName asmName = asm.GetName();
int iterations = 150000;
AssemblyLoadContext[] alcs = new AssemblyLoadContext[iterations];
for (int i = 0; i < alcs.Length; i++)
{
    alcs[i] = ...
}

Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for (int i = 0; i < alcs.Length; i++)
{
    alcs[i].LoadFromAssemblyName(asmName);
}
stopwatch.Stop();

For returning assembly in Load:

alcs[i] = new CustomALC()
...
class CustomALC : AssemblyLoadContext
{
    protected override Assembly Load(AssemblyName assemblyName)
    {
        return assemblyName.Name == asmName.Name ? asm : null;
    }
}

For fallling back to default ALC:

alcs[i] = new AssemblyLoadContext("DefaultALCFallback")

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Issue Details

---

Author:	elinor-fung
Assignees:	-
Labels:	area-AssemblyLoader-coreclr
Milestone:	-

[vitek-karas]

I assume we have to use normal pointer type to pass around the new data structure because it goes through one COM-like interface...
Regardless I think it would be nice if we could make it a reference in all other places - it is stack allocated (only - which is great). Maybe even a const reference if possible.

[vitek-karas]

I love this change - delete bunch of code, remove complexity... and get a perf win ...

[AaronRobinsonMSFT]

@stephentoub based on his love of perf wins

[elinor-fung]

Merging, since the latest commit was non-functional changes and all the CI legs passed before it, so I don't think I need to add to the current PR/CI backlog.