Add Map.filterMap & fixed #151

[pbiggar]

Thanks! Should be able to get this merged easily enough:

• needs tests in both rescript and native
• the .mli files need to match (rescript-build-and-test-9.1.4 is failing as a result)
• code needs to be formatting (use npm run format I think)

[pm5]

Hello there, fixed the formats and mli and added some tests. Not sure what failing in build and documentations though.

[pbiggar]

Hello there, fixed the formats and mli and added some tests. Not sure what failing in build and documentations though.

Thanks! To fix the documentation, I think you just need to run make doc

[pm5]

Hello there. Finally had some time to address this :)

[pbiggar]

My apologies, I should have merged this before splitting the repos apart. We'll need to move this to the other repos