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.
This moves all the basic random value generation into the Rand instances for
each type and then removes the `gen_int`, `gen_char` (etc) methods on RngUtil,
leaving only the generic `gen` and the more specialised methods.
Also, removes some imports that are redundant due to a `use core::prelude::*`
statement.
1. Fail when there's no package script and no crates named
main.rs, lib.rs, bench.rs, or test.rs.
2. Inject the crate link_meta "name" and "vers" attributes, so
that the output file gets named correctly in the library case.
3. Normalize '-' to '_' in package names.
Pulled out tests into their own modules inside the files they test,
as per the draft style guidelines.
Started a new module, path_util, for utility functions to do with
paths and directories.
Changed default_dest_dir to use a condition and return Path
instead of Option<Path>.
