Add char-pair literals

[lynn]

⁽ strings eat up all characters in the space-through-tilde ASCII range:

    $ ./jelly eun ⁽HelloṚ
olleH

⁾ strings are two characters long:

    $ ./jelly eun ⁾abx5
aaaaabbbbb

[lynn]

Dennis wrote that he intends ⁽ to work slightly differently from this PR’s implementation:

@Lynn Hm, ⁽ isn't quite what I had in mind. For example, ‘o' + 128 = ‘ṁ’. Then ⁽HellṁX would pseudo-randomly choose a character from the string Hello.

But then:

Still have to decide whether the unprintable code points should correspond to those ASCII characters (rarely useful except for a handful of them) or use a custom encoding of all Unicode characters.

So this feature will probably have to wait a little.