Add Core.Partial

[ndmitchell]

Should reexport the nice Partial stuff, plus some partial functions, e.g. fromJust.

[vincenthz]

I think for generic combinator like isJust or isLeft it make sense to have them explicitly only from importing Core.Partial. Otherwise collection stuff like head or last should be available as part of the Sequential (automatically by importing in Core.Collection).

Otherwise everything else is available through Core, and there's now a specific PartialError instead of the normal async "error" call