ena 0.1.0

Union-find, congruence closure, and other unification code. Based on code from rustc.
Documentation
An implementation of union-find / congruence-closure in Rust. Forked
from rustc for independent experimentation. My intention is to iterate
and improve this code and gradually bring back changes into rustc
itself, but also to enable other crates.io packages to use the same
code.

### Name

The name "ena" comes from the Greek word for "one".

### License

Like rustc itself, this code is dual-licensed under the MIT and Apache
licenses. Pull requests, comments, and other contributions are assumed
to imply consent to those terms. Moreover, it is understood that any
changes here may well be used in rustc itself under the same terms.