Implement clamp

[jacg]

It would be useful to have uom support clamp.

My attempt at implementing it ran into problems caused by there being different definitions for

• {std,core}::cmp::Ord::clamp
• {f32,f64}::clamp

and my not knowing how to treat these separately inside uom's implementation.

[iliekturtles]

Closing this as a duplicate of #239.

• Want to open a draft PR with the code you trying?
• That issue discusses MSRV issues. At this point I'm inclined to bump to 1.50 with 1.64.0 coming out today.

[jacg]

Ah, sorry, I should have found #239.

Here's the draft PR: #377.