Introduce `lockable' facility.

	* src/misc/lockable.hh: New file.
	* src/misc/Makefile.am: Distribute it.
	* src/inst/text_label.hh, src/inst/exp.hh, src/inst/label.hh,
	* src/inst/int_exp.hh, src/inst/int_exp.cc:
	Use lockable.
	* dev/inst-solver-gen.py: Update accordingly.