Investigate whether to add a fixed-point type

[abizjak]

Task description

It might make sense to have a type to represent fixed-point fractional values and support calculations on it as part of concordium-std.

The design needs to be well thought-out to satisfy at least the following

• efficient at runtime. Fixed point computation are for the most part just computation with integers so we should not be needlessly wasteful.
• type safety
• correct and well-documented rounding behaviour. Failure should be explicit.

Sub-tasks

• [ ] Investigate the design space and make a proposal.
• [ ] Implement the API and test.
• [ ] Analyze performance, code size of generated code.