Use `opam pin` for development.
opam: update depencency lists
Merge branch 'master' into opam
opam: remove unused ocamlgraph dependency
opam: add tdk dependency
opam: update README
opam: expand test matrix
opam: use version-specific opam-street
Merge pull request #369 from frenetic-lang/opam

Use `opam pin` for development.