Consider providing templates implementing other ways of expressing fixed points

[Pharap]

There are times where it may be useful to be able to represent fixed points using notation other than the conventional Q format.

For example, it may be useful to have a type expressed as Fixed<sign, integer, fraction>, where:

• Fixed<0, integer, fraction> is implemented with UFixed
• Fixed<1, integer, fraction> is implemented with SFixed
• All other representations are undefined

Other notations include:

• s:m:f format, as described above as Fixed<sign, integer, fraction>
• fx format where fx1.16 describes a 16-bit value with 1 integer but and 15 fractional bits.

This isn't a particularly important feature, but there are cases where it might be useful.

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There's the possibility of implementing it through template 'trickery' and type aliases or inheritance, but there's also the possibility (should the Fixed approach turn out to be more convinient) of inversing things such that UFixed and SFixed become inheritors of the true implementation with minimal changes to the API such that it could still be compatible for more abstract use cases.

Such an eventuality is probably still a long way down the road and may not be worthwhile, but it's at least worth pondering for the time being.