Decimal floating-point

Implement IEEE 754 Decimal128 (and possibly Decimal64 as well, though it's less important). This has a dependency on [fixed-width integer types](https://github.com/apple/swift-numerics/issues/4) because we want to be able to leverage UInt128 arithmetic for the implementation.

Start by reading the fixed-width integer types dependency in issue #4, since the proposed implementation depends on UInt128 arithmetic. Define the IEEE 754 Decimal128 scope first, including whether Decimal64 is included; done means the selected format is implemented with suitable validation against the specification and relevant arithmetic tests.

Written by the indexing model from the issue text.