Remove unnecessary TODOs

- GEP/extract chaining is optimised by LLVM already.
- We can create an alloca for array indexing, and
  LLVM will optimise it away if the array is already
  addressable.
- Interface change optimisations may make sense, but
  this is a very premature optimisation at this stage.
Implement panic(), recover() builtins
Merge pull request #133 from axw/libgo-panicbuiltin

Implement panic() and recover() builtins