Rollup merge of #134063 - tgross35:dec2flt-refactoring, r=Noratrieb

dec2flt: Clean up float parsing modules

This is the first portion of my work adding support for parsing and printing `f16`. Changes in `float.rs` replace the magic constants with expressions and add some use of generics to better support the new float types. Everything else is related to documentation or naming; there are no functional changes in this PR.

This can be reviewed by commit.

Author: jieyouxu