Add proper floating point printing support.
Merge branch 'format-float' of github.com:dhylands/micropython into dhylands-format-float
Rename formatfloat file; remove MICROPY_ENABLE_FLOAT from mpconfigport.h.

MICROPY_ENABLE_FLOAT is automatically set in mpconfig.h if MICROPY_FLOAT_IMPL
is set to a non-zero value.