Add freelist to environment

[ncloudioj]

This allows the consumer to query the total # of pages on the freelist for each LMDB environment, then they can use it with env.info() to precisely check the space usage of that environment.

Note: the approach that I used to query the freelist was not documented in LMDB but a direct copy from its utility mdb_stat. I won't feel bad if we decide to not use the uncharted stuff from LMDB. 😉

[mykmelez]

Note: the approach that I used to query the freelist was not documented in LMDB but a direct copy from its utility mdb_stat. I won't feel bad if we decide to not use the uncharted stuff from LMDB. 😉

I'm ok with this, although it'd be useful to document it in a doc comment on the function in question.

This also makes me wish for rust-lang/rfcs#1491, so we can identify this function as unstable. Although I guess all functions exported by this crate are potentially unstable, given its pre-1.0 status.

[mykmelez]

Nit: the only other thing I'd add are some hints about how this API could be used.

[ncloudioj]

Thanks!

Added more document to describe the usage and potential use cases.