core: Comments only -- move FIXME to correct location
rustpkg: update README saying which tests pass
rustpkg: Make path searching work as described in the rustpkg doc

rustpkg now searches for package directories in ./src rather than
in . . I also added a rudimentary RUST_PATH that's currently
hard-wired to the current directory. rustpkg now uses src/, lib/,
and build/ directories as described in the manual.

Most of the existing test scenarios build now; the README file
(in a separate commit) explains which ones.
rustpkg: Correct directory structure in test scenarios
rustpkg: Preliminary work on install command

Mostly just tests (that are ignored); install command is still
stubbed out.
auto merge of #6054 : catamorphism/rust/rustpkg, r=graydon

r? @graydon

Sorry, this pull request is a few different things at once, but I tried to make them separate commits.

First, as before, this should do file searching the way that's described in the doc now.

Second, there's also some preliminary work on the install command (really just tests for it).