Explicitly stated important optimizations for V

V relies on these three optimizations performed by gcc:

`-finline`
Macros are replaced by inline functions where possible.
This leads to nice features such as type checking and syntax
safety. However, if the inline option is not specified, these
functions lead to significant runtime bloat.

`-foptimize-sibling-calls`
Most of the finite-state machines which implement the various
lexing parts of V use lookup tables of function pointers. Without
tail-recursion, this can lead to explosive increase in stack usage.

`-freg-struct-return`
The var_t type is designed to fit into 64bits. This has several
advantages such as allowing all variables to be passed around in
registers. While excluding this option would not break anything,
variables are directly passed in most of the code and would cause
some runtime inefficiency.

These optimizations are already enabled with the `-O` flag, but
explicitly stating them allows their use in debug builds and indicates
their importance to the code base.