This provides some deduplication since most contexts are used multiple
times.

Also, store singleton contexts directly in the Value object. This
saves a 16-byte Context object. This is useful because the vast
majority of contexts are singletons, e.g. 23723 out of 26138 in a
NixOS 19.03 system configuration.

The vast majority of strings are < 16 bytes, and so can be stored
directly in a Value. This saves a heap allocation and an indirection.

This allows strings < 23 characters (up from 16) to be stored directly
in Value. On a NixOS 19.03 system configuration evaluation, this
allows 1060588 out of 1189295 (89%) strings to be stored in Value.