Add support for Rust lists and Python dicts as literals in initializers

[lhstrh]

It that seems having list expressions as first order citizens in LF as proposed by @oowekyala in #492 is likely to cause more issues than they would solve. For the time being, we can just escape lists in Rust initializers with {==}. In the long run, however, we should support lists (and also Python's dicts) directly in our syntax and treat them as lliterals. It is likely that all we'd need to do (in the grammar) is change the definition of Value

Originally posted by @lhstrh in #492 (comment)