These constants can be useful outside of their current module. Make them
`pub(crate)` to allow for this.

Fix or elaborate existing float parsing documentation. This includes
introducing a notation.

This module currently contains two decimal types, `Decimal` and
`Number`. These names don't provide a whole lot of insight into what
exactly they are, and `Number` is actually the one that is more like an
expected `Decimal` type.

In accordance with this, rename the existing `Decimal` to `DecimalSeq`.
This highlights that it contains a sequence of decimal digits, rather
than representing a floating point decimal as would be expected.