Add fromSetMaybe and fromSetMaybeA for maps

[meooow25]

These are like the existing fromSet and fromSetA, but allow dropping
some keys from the input set. These functions will also be used by the
in-progress Map-Set merge API.

Also adjust fromSet and fromSetA docs to make it consistent with the new
functions.

Closes #1232

[meooow25]

Pulling g out of buildTree and inlining it seems to have improved the performance of fromSetA. I made the change to only share the code with fromSetMaybe, so this is a nice surprise.

Name                     Time - - - - - - - -    Allocated - - - - -
                              A       B     %         A       B     %
Lazy.fromSetA inner      863 μs  753 μs  -12%    2.4 MB  1.9 MB  -21%
Lazy.fromSetA outer      678 μs  578 μs  -14%    1.9 MB  1.4 MB  -26%
Strict.fromSetA inner    1.1 ms  774 μs  -26%    3.1 MB  2.1 MB  -31%
Strict.fromSetA outer    706 μs  477 μs  -32%    2.0 MB  1.0 MB  -49%
fromSet                  555 μs  560 μs   +0%    1.4 MB  1.4 MB   +0%