Replace String with Rc<str> or other data structure for tracking names in parser and checker

[johnchandlerburnham]

Currently we do an egregious amount of cloning of String names, like in Lam/Slf/All/Let binders, Vars and Refs. But the thing about names is that they are immutable, once we parse we should never need to change them. So a datatype like

struct Name(Rc<str>)

might significantly improve performance here. Doesn't have to be exactly Rc<str>, but that's probably a good place to start investigating.

Also relevant to argumentcomputer/yatima#38 since I believe String isn't in sp_std