Allow importing symbols into current module namespace

Currently given foo.x which contains

pub const var1 = ..;
pub fn fun1() -> ...;

one do the following
import foo;

const var1 = foo::var1;

to import var1 into the current module's namespace. However this is clumsy and does not allow the same to be done for fun1. it would be nice if dslx supported "use" from rust.

https://doc.rust-lang.org/rust-by-example/mod/use.html

This is also related to https://github.com/google/xls/issues/352