This commit makes the formatted values' main constructors `const`,
so that they can be used in constant definitions.
`from_usize` constructors can't be made const because `try_into`
can't be used in `const` functions as of rustc 1.64.0. Same for
`Percent::from_ratio`: floating point arithmetic is not allowed.
Introduce a set of Constructor traits which are implemented on
integer of float depending on the formatted values. This traits
allows building formatted values using expressions such as:
```rust
let buffer_nb = 20.buffers();
let size = 42.k_bytes();
let duration = 15.minutes() + 30.seconds();
let quantity = 42.default_format();
let progress = 20.percent();
let progress = 0.2.percent_ratio();
```
... users would be able to bypass the range checks and build a
defined Rust value which would be interpreted as `None` in C code.
Added format module examples for formatted values constructions.
Some operations were implemented on types that wouldn't result in
the expected physical unit. E.g.:
- `ClockTime / ClockTime` results in a unit-less factor.
- `u64 / ClockTime` would result in a `1 / ClockTime`. Since we don't
use any `Frequency` type, this operation is removed. Users should
use the `ClockTime` accessors to compute the expected value.
This commit also adds:
- multiplications with integers as the left hand side operands.
- `Partial{Eq,Ord} for `Signed<T>` with `T` as left hand side operand.
- `opt_add` / `opt_sub` for `Signed<T>` with `T` as left or right hand
side operands.
- missing tests for `Partial{Eq,Ord}` and `OptionOrd`.
This implementation can interfere with unrelated code and was removed:
- `Signed<usize>.` `PartialOrd` makes existing code computing the len
of slices needing type annotation because the len is later used in
a comparison for which the compiler is unable to determine if
the len is `Signed<usize>` or `usize`.
