Use custom function instead of 'atoi' to read 32-bit integers.

Use custom function instead of 'atoi' to read 32-bit integers.

Using 'atoi' displeased me for several reasons:
    - 'atoi' doesn't check for overflow
    - by the time 'atoi' was called, re2c has already parsed input
      string and knows that it is well-formed, no need to do it twice
    - atoi returns 'int', custom function allows to refine types and
      avoid casts between signed/unsigned values or [-Wsign-conversion]
      warnings

Added test for new conversion functions.