Adds these extension methods on CodeInstruction:

- LocalIndex, which returns the operand of any ldloc/ldloca/stloc, even
  the short forms or the fixed forms (i.e. ldloc.s or ldloc.1)

- ArgIndex, which does the same but for ldarg/ldarga/starg

Adds these static helper methods to CodeInstruction:

- LoadLocal, which allows you to produce an optimal load from an index,
  optionally loading the address

- StoreLocal, which allows you to produce an optional store to an index

- LoadArg, which allows you to produce an optimal load from an argument,
  optionally loading the address

- StoreArg, which allows you to produce an optimal store to an argument

All of the methods are annotated with doc comments and include links to
their counterparts via `<seealso>` references.

This adds four new static methods to CodeMatch:

- LoadsLocal which matches any local load (ldloc family)
- StoresLocal which matches any local store (stloc family)
- LoadsArgument which matches any argument load (ldarg family)
- StoresArgument which matches any argument store (starg family)

Matching specific indices can be done with the predicate and checking
the instruction LocalIndex or ArgIndex

I'm having trouble getting feedback on this one but I think this is
what we want?