implement `From` for `TRS` and UTRS

Right now, `TRS` is declared as:

``` rust
pub struct TRS {
    pub(crate) lex: Lexicon,
    pub(crate) utrs: UntypedTRS,
    pub(crate) ctx: TypeContext,
}
```

It would be nice to have a public way to get an untyped TRS from a TRS.

``` rust
impl From<TRS> for UTRS
```

would be a straightforward to go about doing so.