Distinguish string values from vector values transformed by quasiquote macro

[radhikalism]

This behaviour seems to be unexpected:

(quasiquote "str")

(#\s #\t #\r)

String arguments passed to transform-quasiquote-arguments satisfy vectorp, so are handled as vectors. Instead, passing through string values as in the patch yields:

(quasiquote "str")
"str"

[dan-robertson]

I suppose this was fixed in #100.