lisp: split lisp into lisp and sys

Generic definition of tagged pointers, along with simple
constructor/destructors should be in lisp.h for use by other headers.

sys.h on the other hand contains all the general system methods.

Makefile

@@ -19,7 +19,7 @@ CFLAGS=$(GFLAGS) $(DFLAGS) -DTEST_VERBOSE=1
 endif
 
 # Units to compile
-UNITS=src/sv.c src/vec.c src/stream.c src/symtable.c src/tag.c src/lisp.c src/reader.c
+UNITS=src/sv.c src/vec.c src/stream.c src/symtable.c src/lisp.c src/sys.c src/reader.c
 OBJECTS:=$(patsubst src/%.c, $(DIST)/%.o, $(UNITS))
 
 TEST_UNITS=test/main.c