Add keyword avoidance mechanism for F* backend

[protz]

Right now the compiler makes it possible to generate invalid F* code, because some valid identifiers in hacspec (e.g. open) are keywords in F*, which then results in invalid code such as let open = ....

Here's the list of keywords in F*: https://github.com/FStarLang/FStar/blob/master/src/parser/FStar.Parser.Lexhelp.fs#L140