The previous commit renamed `Decimal` to `DecimalSeq`. Now, rename the
type that represents a decimal floating point number to be `Decimal`.
Additionally, add some tests for internal behavior.
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 base-10 floating point (decimal) number.
Additionally, add some tests to validate internal behavior.
