Fix/supertype calculations

[LlamaLad7]

The first commit is just a tiny bug in Mixin's local calculations that causes a hard crash if triggered.
The second commit is more interesting:
I forced Mixin's LVT generation on every method in every mixin target class on AOF7 and it failed on about 12 occasions with various exceptions, including 1 NPE that occurs naturally in the pack, and 4 attempts to live-load minecraft classes (!), albeit only on AppClassLoader so it didn't work anyway. The issues are largely related to attempts to look up ClassInfos for array types, which do not exist.
My overhaul includes:

• An isSubTypeOf implementation largely taken from upstream ASM which has recently had many fixes to SimpleVerifier
• An isInterface implementation that no longer throws on array types
• isAssignableFrom and merge implementations based on a bespoke getCommonSupertype from MixinExtras' Expressions (the base implementation offloads most of this work to reflection which we cannot do)
• An explicit getClass override to guard against accidental live-loading
• I also removed the special-casing for the current class since it offers no real performance benefit and complicates things.

After these changes there are no errors at all.

[LlamaLad7]

I have since tested LVT generation on every loaded class in AOF7. The original implementation produces many hundreds of errors but my one still has a handful so I will investigate those.

[LlamaLad7]

Okay, all remaining errors are just the LVT generation correctly failing when it encounters a class that doesn't exist.

[LlamaLad7]

Minor fix to my assumption that arrays could not implement interfaces.