Normalize hyphens to underscores in crate names

Normalize hyphens to underscores in crate names

This change allows *packages* to have hyphens in them, but they are always
translated to underscores when translated to a crate name. This means that all
crates are compiled with a `--crate-name` that has no hyphens (as well as
`--extern` directives having no hyphens).

Binaries, examples, benchmarks, and tests, however, are allowed to contain
hyphens in their name. The "crate name" will still have an underscore, but the
output will be in the same dasherized name.

Explicitly named targets are not allowed to have hyphens in them as well.