All of the modules in the standard library were just straight reexports of those
in libcore, so remove all the "macro modules" from the standard library and just
reexport what's in core directly.

This mirrors the same hierarchy in the standard library.

This commit stabilizes the prelude::v1 module of libcore after verifying that
it's a subset of the prelude of the standard library with the addition of a few
extension traits.

Stop using stability to hide the implementation details of ParseFloatError and
instead move the error type into the `dec2flt` module. Also move the
implementation blocks of `FromStr for f{32,64}` into `dec2flt` directly.