Rename FixedDecimalFormatter to DecimalFormatter

[robertbastian]

It now accepts SignedFixedDecimal, but SignedFixedDecimalFormatter is a mouthful. Other components have names that are more aligned with the component names than the inputs as well.

[sffc]

I named it FixedDecimalFormatter in case we wanted to reserve DecimalFormatter for the kitchen sink (#275).

[Manishearth]

Start 11:12

• @sffc We can't really discuss the renaming until we decide how many formatters to have. We'll have at least 6-7 composite decimal types. Do we have separate formatters for them? Do they have different payloads?
• @younies I think we should have fewer formatters. DurationFormatter needs both of them.
• @Manishearth The benefit of splitting the data does not really justify the cost of having more types. At most we should have multiple constructors.

Tentative conclusion:

• One formatter type for WithNan<WithInfinity<SignedFixedDecimal>>
• Probably keep scientific in a different type
• Maybe have a new payload inside the formatter based on implementation details, and optimize it in various ways
• Do not have multiple constructors that put the formatter in funny states