Improve performance of basic parser methods
Improve performance of basic parser methods

- I modified the basic integer methods (.eat_u8(), .eat_u16(), etc.)
  to perform fewer EOF checks.
- I modified the basic integer methods (.eat_u8(), .eat_u16(), etc.)
  to use Vec::get_unchecked(), a method that skips bounds checking.
- I modified the atom(), atom_utf8(), string(), and binary() methods
  to use .iter_mut() rather than Vec::push(); this requires the use of
  an unsafe method, Vec::set_len().

On my machine, the time to parse a 100 MB bert file dropped from
~450ms to ~270ms.

gnuvince/ppbert,gnuvince/ppbert