Local and core ingot support.

[g-r-a-n-t]

This PR implements the following:

• resolution of ingots on disk with error handling
• folder ingot checking (e.g. fe check my/ingot/path)
• a core ingot that is statically loaded by default and overridable using the --core arg (e.g. fe check my/ingot/path --core my/core/ingot/path)

[g-r-a-n-t]

@sbillig there are still a few minor things that I want to address on this PR, but it's pretty much done

[sbillig]

Looks good. Left a couple minor nitpicks. You'll probably run into #1049 if you add real stuff to core and try to use it.