Issue 5113/2: Parser: discriminate quoted and non-quoted strings

Issue 5113/2: Parser: discriminate quoted and non-quoted strings

Checking quoted strings for syntactic correctness seems spurious, as does
splitting them into multiple symbols.  Now "x.y" is treated as a list
of two symbols without the quotation marks, but as a single string or
symbol when given quotation marks.

This allows using quotation marks for creating arbitrary symbols, and
consequently specifying key lists using quoted strings without the
strings being split in other components or being rejected because
of syntactical reasons.