Represent implicits as a tree instead of a list to make conjoining them constant time

[nikswamy]

With @mtzguido, we noticed that concatenating implicit argument lists makes type checking large expressions quadratic, as reported by @jaylorch and @LukeXuan

This PR represents implicits as a tree, enabling a constant time operation to conjoin them, coercing them to a list only when needed eventually to iterate over all implicits.

[mtzguido]

I wonder if this will also improve the performance of the long lists in HACL*. @R1kM, @msprotz , could you remind me which module that was so I check it?

[nikswamy]

I think it's things like Test.Vectors.Chacha20Poly1305.fst that take a long time to check. I think this PR will help, but those list literals are also written as polymorphic instantiations of Cons.

If it were to be written as

let cons_u8 = Cons #u8`
let nil_u8 = Nil #u8

let l = cons_u8 0x9buy <| cons_u8 0x18uy <| ...

I would expect to see significant speedups