Add variable-entry-size LRU cache implementation in internal/memcache

Created by: efritz

Add internal/memcache, a variation of github.com/hashicorp/golang-lru that allows cache values to report their size. This allows an in-memory cache to be gauged on its memory pressure rather than assuming that all cache values are equally weighted.

This will be necessary to store variable-length documents in the precise-code-intel-bundle-manager, which is being rewritten in Go (effort tracked in https://github.com/sourcegraph/sourcegraph/issues/9964).

This PR is being submitted separately to get more in-depth reviews on the cache implementation (which could be used easily in other parts of the code base) without getting overshadowed by larger changed in the porting effort.