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 value pointers of lists with 1 or 2 elements are now stored in the
list value itself. In particular, this makes the "concatMap (x: if
cond then [(f x)] else [])" idiom cheaper.
Code that links to libnixexpr (e.g. plugins loaded with importNative, or
nix-exec) may want to provide custom value types and operations on
values of those types. For example, nix-exec is currently using sets
where a custom IO value type would be more appropriate. This commit
provides a generic hook for such types in the form of tExternal and the
ExternalBase virtual class, which contains all functions necessary for
libnixexpr's type-polymorphic functions (e.g. `showType`) to be
implemented.
