More friendly conversion using `From` trait.

[katyo]

I like this crate mainly because it support arbitrary exponent which is not constrained only 0..base^bits range.

But weak side is missing developer friendly conversion methods.
I mean two different aspects here:

1. Conversion between core rust numeric types such as u*, i*, f* and Fix type.
2. Conversion between Fix types with different bits, base and exponent.

I think in this case it is pretty usable to implement core rust conversion traits: From, TryTrom, Into, TryInto.

[katyo]

I trying fix this here.