unicode names in language objects

[moodymudskipper]

The quote(`\U{123}`) syntax is illegal. This means quote() cannot be used as a constructor when a language object contains some characters, depending on unicode_representation.

We could use substitute() or quote(), but only once, if we nest those it gets messy, and with NSE we'll have more issues.

I think we need a "as.call" constructor for language, and we fall back to it when we can't use quote.
as.call() cannot create symbols though, so we need to have a proper constructor for symbols, that would default on quote as well, but we'd have also "as.symbol" and "as.name", maybe "sym" (and then add "expr" to the language constructors, falling back to quote if contains !!, !!, {{).