Add `Symbol::{with,with2}`.
Add `Symbol::{with,with2}`.

And remove the `unsafe` blocks, they're not necessary.

Also rewrite `InternedString::{with,with2}` to use the new functions.

Finally, add some comments about the speed of the
`as_str()`/`as_interned_str()` functions.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust