specialize and accelerate gv_trim_zeros

specialize and accelerate gv_trim_zeros

This function trims unnecessary trailing zeros from a printed floating-point
number. It was written to be extremely general, however it is only ever used to
trim a number printed with the format string "%.02f". We can take advantage of
this fact to know that, if it can locate a period, there are exactly two digits
following this that need to be checked. This then allows implementing the
remainder of the function not as a loop but as simply a few branches.

Using tests/regression_tests/large/long_chain, which has been used for other
profiling in this area, this drops total executed instructions from 8160952787
to 8143275099, a speed up of ~2%.