Simplify unique_string construction.

	* src/misc/unique_string.hh, src/misc/unique_string.cc (create):
	Remove. Move its implementation to unique_string's constructor,
	hence make unique_string aggregate a pointer-to-string instead of a
	reference.
	* src/misc/test-unique_string.cc, src/vm/virtual_machine.cc, 
	* src/shell/shell.cc, src/parse/asm-parse.yy.gen.py,
	* src/parse/asm-scan.ll.gen.py, src/inst/label_exp.hh,
	* src/inst/section.hh, src/inst/label.hh:
	Adjust.