Migrate away from libnethost.lib

[alexrp]

The only reason #5 is blocked is our use of the MSVC ABI (see #5 (comment)). We use that because we link to libnethost.lib. If we could avoid doing that, then 32-bit support would be unblocked, and it would be much easier to build Ruptura on non-Windows platforms.

[alexrp]

I see 2 paths forward here:

1. Just roll our own logic equivalent to get_hostfxr_path(). Or, at least equivalent enough for our purposes.
2. Copy the relevant source files from the .NET repository and build them as part of the module project.

I think I prefer the former.

[alexrp]

Third option: Just find hostfxr.dll from Vezel.Ruptura.Injection (potentially using Microsoft.Extensions.DependencyModel) and pass the path along to the module. Much easier to implement, less native code to maintain, and early error diagnosis if it's missing. This is what we'll do.