suggest: implement caching

[mdempsky]

In generating suggestions for cmd/compile/internal/ssa, the two big time overheads are:

1. Parsing the sibling source files within the package
2. Loading imports

If we could cache these results, then we'd be on par with nsf/gocode for suggestion performance.

[mdempsky]

In the cache branch, I've prototyped caching for both of these. Suggestions within cmd/compile/internal/gc now take on the order of 30ms to 40ms with a warm cache.

[OneOfOne]

I've been using this for a few days now (with both go 1.10 and master), I didn't run into any issues and does seem as fast if not faster than nsf's.

[OneOfOne]

Any chance this could be merged to make it easier to pull from editors that use this gocode fork?

[OneOfOne]

Using -source without this branch is pretty much unusable on a big'ish repo.

[abennett]

Let us know what support we can provide.

[phenixrizen]

#51 Implements a cache and solves this issue

[charlievieth]

See: #55 and #54 as well.

[stamblerre]

Cache branch was merged in #71.