rust-cpython: binding for headrevs()
This uses the core `dagops::retain_heads` to give a Rust implementation
to `mercurial.dagop.headrevs`.
Testing happens for now from `test-rust-ancestors.py`
(for quick and minimal change), but it'd made more sense to put the binary
index data elsewhere and to create a new test python module
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}