Add =~ and !~ operators on strings

`left =~ right` return true if left contains right, using Rust's
`String::contains`. `!~` is the negated version.

A new `apply_operator` function is added which decouples evaluation from
`Value::compare`. This returns a `Value` and opens the door to
implementing `+` for example, though it wouldn't be useful immediately.

The `operator!` macro had to be changed slightly as it would choke on
`~` in arguments.

crates/nu-parser/src/lib.rs

@@ -1,5 +1,4 @@
 pub mod commands;
-pub mod debug;
 pub mod hir;
 pub mod parse;
 pub mod parse_command;