Use MethodHandle lookups instead of reflection generally

[prdoyle]

Reflection is awkward for non-public methods and classes, but MethodHandle lookups can function fairly naturally on non-public things. Bosk functionality that uses reflection should allow MethodHandles.Lookup objects to be supplied so that non-public things can be used.

Note though that I think we should still avoid using private things. I think it's important that developers can mark something private and know that they retain full latitude to modify or remove it later without breaking anything.

[prdoyle]

MethodHandles.Lookup.dropLookupMode works such that when you drop PRIVATE you also lose PROTECTED. I think I can live what that: PROTECTED is probably the least-used mode, and being unable to use those is probably not a hardship.

[prdoyle]

Turns out I'm going to enforce the PRIVATE restriction myself anyway.

A Lookup without private access can't even see the methods in question to determine whether they're malformed. If we try to implement it this way, any time anyone accidentally marks a method as private, is just quietly stops participating.

[prdoyle]

Did hooks in #264.