Add ldexpf16, ldexpf128, scalbnf16, and scalbnf128

[tgross35]

The scalbn functions are similar enough that they can easily be made generic. Do so and f16 and f128 versions.

[tgross35]

f16 and f128 versions are untested, blocked until I can figure out how to do that with MPFR.

Edit: done in #392. I cherry picked that commit here but that one should probably merge first.

[tgross35]

I think the f16 version is running into the same issue as #419 (comment) (among other issues) so I am just going to drop that for now, keeping only scalbnf128.

[tgross35]

Oh, interesting; the same problem seems to be showing up for the f128 version:

thread 'mp_edge_case_scalbnf128' panicked at crates/libm-test/tests/multiprecision.rs:17:48:
called `Result::unwrap()` on an `Err` value:
    input:    (0x00000000000000000000000000000003, -2) (0x00000000000000000000000000000003, 0xfffffffe)
    expected: 0x00000000000000000000000000000000 0x00000000000000000000000000000000
    actual:   0x00000000000000000000000000000001 0x00000000000000000000000000000001

Caused by:
    ulp 1 > 0

Testing with the new (unreleased) version of Rug returns 0x00000000000000000000000000000001, so I will just wait for that release.

[tgross35]

f16 needs way more rounds than the other operations, I am going to check if there is a better algorithm.