Alphanumeric variable names are not enforced

The documentation states that only A-Z, a-z, 0-9, and _ should be accepted in the variable names during expansion. Ion doesn't seem to enforce this.

Reproduction:

let ❲┛ಠДಠ❳┛ = ┻━┻
echo $❲┛ಠДಠ❳┛

Build information:

rustc -V: rustc 1.33.0-nightly (4c2be9c97 2019-01-22)

git rev-parse HEAD: 918e1fc4