Make 0.20.x version compatible with funty 1.2

[ed255]

Hi! Recently I tried to introduce a new dependency into a project I'm working on and encountered this issue: https://github.com/bitvecto-rs/bitvec/issues/105 Unfortunately I didn't find a way to solve this dependency compilation failure in my project, as we already have another dependency that requires bitvec 0.22.x which requires funty 1.2. The new dependency I want to introduce requires bitvec 0.20.x which isn't compatible with funty 1.2; and it seems it's not possible to have funty 1.1 and funty 1.2 at the same time.

This PR is an update from https://github.com/bitvecto-rs/bitvec/pull/110 but for the latest release in the 0.20.x version. It makes bitvec 0.20.x work with funty 1.1 and funty 1.2. I've also updated the version in Cargo.toml to 0.20.5 and would like to ask if you could publish this release to crates.io.

I didn't find any branch for the 0.20.x version, so I made the PR agains develop, but it obviously is not compatible. Is there a better way for me to submit a PR for the 0.20.x branch?