Do not trust *read-default-float-format*

Note that this side-effects the compile-time environment, which is unclean.

Alternatives:
1- modifying all the constants in the code to explicitly use an f0 suffix.
2- defining an :around method for ASDF to ensure *r-d-f-f* is always 'single-float
3- campaigning the current ASDF maintainer (not me) so he can guarantee the binding.