catch missing foreign functions earlier

[luite]

currently when some package has ccall imports that are not implemented in shims, GHCJS does not complain about any missing imports, but you get a runtime exception about undefined symbols, this is not a great user experience (especially since there are so many packages out there, and it's often unclear which packages use foreign code)

Proposal:

• Keep track of all ccall imports per module, save a list in the .js_o file
• When linking shims, build a list of supplied symbols for each .js file (keep these in a cache)
• Print a warning about all missing symbols
• Add flag to avoid printing these warnings

Limitations:

• Works only per module, not per package (I don't think this is a problem, although it makes it harder to check that everything in a package is shimmed)
• Either limited to some particular styles of declarations (since accurately detecting top-level symbols provided would require running the code), or requires some extra annotation syntax to indicated what symbols are supplied

[luite]

Doing this per module is problematic: Due to inlining, foreign imports non-imported modules might show up in the generated code. Probably better to track this per package.