Compile 'do' as a sequence of expressions in Ruby.

This allows the compiler to compile (do Exp1 Exp2) properly when Exp2 is in tail position so that it can be tail-call optimized as expected.
Add tests that check that the `do` optimization maintains the correct behavior.
Add comment explaining that compile_do exists to allow TCO when `do` is involved.
Merge pull request #1 from tizoc/compile-do

Compile 'do' as a sequence of expressions in Ruby.