compiler: optimize zero and one item quasiliterals.

* share/txr/stdlib/compiler.tl (expand-quasi): Do not emit
sys:fmt-join call unconditionally. If expand-quasi yields
a list of one expression, we can just yield that expression.
If the list is empty, we can yield a mutable empty string.
(That case will not arise via `` because that converts to ""
at read time, but code that generates quasiliteral syntax
might have an empty case, and expect a mutable string in
all cases).

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