context.hpp: modified
	environment-example.cpp: written test
	environment.hpp: Added environment::resolve_and_clear
	semantics-test.cpp: modified
	modified:   context.hpp
	modified:   environment.hpp
	new file:   examples/context-example.cpp
	new file:   examples/context-example.hpp
	renamed:    environment-example.cpp -> examples/environment-example.cpp
	renamed:    environment-example.hpp -> examples/environment-example.hpp
	renamed:    global-example.cpp -> examples/global-example.cpp
	new file:   examples/semantics-example.cpp
	new file:   examples/stream-interpreter-example.cpp
	new file:   tests/GNUmakefile
	renamed:    semantics-test.cpp -> tests/semantics-test.cpp
Comments and documentation.
Comments and documentation.
Minor modifications.
Changed variable names, some comments, etc.
Cosmetics. Changed template and struct names. builtin -> expr_builder
Minor modifications.
Various changes.

Moved closure template to its own file.
Simplified std_environment template.
Added enter(closure_type&)
Added k_machine_emulation machine and semantics.
typedef de_bruijn<BODY> body_type;