Add infrastructure for builtin predicates

[jacobdweightman]

Allium's builtin types (Int and String) are not very useful right now, because there are no predicates that take them as arguments. Because their representations are opaque (they are basically C++ int and std::string, respectively), we will need to provide builtin predicates to work with them. As a starting point, it would be nice to be able to concatenate strings:

pred concat(in String, in String, String)

the idea is that if a concatenated with b is c, then concat(a, b, c). Examples:

concat("a", "b", "ab") succeeds
concat("a", "b", let c) yields (c = "ab")

This issue is primarily about the infrastructure, so don't worry about the non-generality of the predicate. Eventually we will generalize this to also work in this mode:

concat(_, _, in)

Examples:

concat("a", "b", let c) yields (c = "ab")
concat("a", let b, "ab") yields (b = "b")
concat(let a, "b", "ab") yields (a = "a")
concat(let a, let b, "ab") yields (a = "", b = "ab"), (a = "a", b = "b"), (a = "ab", b = "")