Work towards more constant methods

[c410-f3r]

Something I didn't realize in #360, rust_decimal::Error conflicts with CTFE (Constant Time Function Evaluator) because of the variants that use String.

For example, the following should be possible but isn't at the current time.

const FOO: Decimal = match Decimal::try_new(0, 0) {
    Ok(num) => num,
    Err(_) => panic!(),
};

destructor of `core::result::Result<decimal::Decimal, error::Error>` cannot be evaluated at compile-time
the destructor for this type cannot be evaluated in constants

The idea to workaround this limitation is to convert all string errors into error variants starting with the from_scientific_exact function. Technically a breaking change but such a thing already occurred before without a version bump.

Feel free to close this PR if desired.

[c410-f3r]

Not necessary anymore

[paupino]

Given this is merging into the "vnext" version which is intended to be breaking, this seems perfectly fine - in fact, desired! Thank you