Previously all includes were anchored in one global mess of header
files. This moves the includes into filesystem "namespaces" (if you
will) for each sub-package of Nix.
Note: This commit does not introduce the relevant build system changes.
It is considered bad form to use things from includes in headers, as
these directives propagate to everywhere else and can make it
confusing.
types.hh (which is includes almost literally everywhere) had some of
these directives, which this commit removes.
Replaces most uses of `string` with `std::string`.
This came up because I removed the "types.hh" import from
"symbol-table.hh", which percolated through a bunch of files where
`string` was suddenly no longer defined ... *sigh*
This applies the modernization fixes listed here:
https://clang.llvm.org/extra/clang-tidy/checks/list.html
The 'modernize-use-trailing-return-type' fix was excluded due to my
personal preference (more specifically, I think the 'auto' keyword is
misleading in that position).
