This picks a number of changes from the lazy-trees branch.
As it is hand picked, and does not include some other necessary
changes, it does not build. Subsequent commits will fix that.

I have added a couple of comments of my own as well.

Co-authored-by: Eelco Dolstra <[email protected]>

This fixes the double copy problem and improves performance
for expressions that don't force the whole source to be added to the
store.

Rules for fast expressions:

- Use path literals where possible
   - import ./foo.nix
- Use + operator with slash in string
   - src = fetchTree foo + "/src";
- Use source filtering, lib.fileset

- AVOID toString
- If possible, AVOID interpolations ("${./.}")
- If possible, move slashes into the interpolation to add less to the store
   - "${./src}/foo" -> "${./src/foo}"

toString may be improved later as part of lazy-trees, so these
recommendations are a snapshot. Path values are quite nice though.

This allows clever editors/IDEs to discern the path more easily
for Ctrl+Click navigate to functionality, e.g. when building
.?ref=HEAD