This is intended to improve coverage of the test suite.

Previous to this patch, a big-int, float or imag constant was interned
(made into a qstr) and then parsed at runtime to create an object each
time it was needed.  This is wasteful in RAM and not efficient.  Now,
these constants are parsed straight away in the parser and turned into
objects.  This allows constants with large numbers of digits (so
addresses issue #1103) and takes us a step closer to #722.

Used gcov to find some parts of vm.c, runtime.c, obj.c that were not
covered by any tests.  Still need to use gcov more thoroughly.

This is not fully correct re: error handling, because we should check that
that types are used consistently (only str's or only bytes), but magically
makes lot of functions support bytes.