Iteration Base API

[tshort]

This changes the iteration to support the Base API to return Nothing as appropriate. The following example statically compiles:

using StaticCompiler
using StaticTools
using Libdl

function testfun1()
    if startswith(c"foobar", c"foo")
        return 1
    else
        return 0
    end
end

name = repr(testfun1)
filepath = compile_shlib(testfun1, (), "./", name)

# Open dylib
ptr = Libdl.dlopen(filepath, Libdl.RTLD_LOCAL)
fptr = Libdl.dlsym(ptr, "julia_$name")
ccall(fptr, Int, ())

[brenhinkeller]

Tests are passing so I'll merge and add an integration test -- I do have some concern that the whole "if startswith(...)" may be getting optimized out rather than compiled (which is fine here), but if so and this doesn't scale to cases where the content of the strings isn't statically known at compile-time then we can always add startswith/endswith (etc.) overloads that don't depend on Base.iterate