rust-cpython: drop manual management of mutably_borrowed
RefCell::borrow() should guarantee there's no mutable borrow.
As a follow up, maybe PySharedState can be a pure data structure + function.
Most ref-sharing business has already been moved to PySharedRef* and PyLeaked*.
{
// 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)
}