tests/general: Add some more regression tests. Remove the old Makefile, since we use the Mmake file now.
tests/general: Rename *.nl to *.m. Add new test `string_test_2.m'.
tests/* Use mmake. Use main/2 rather than main_predicate/3.
`arithmetic.nl' tests the arithmetic builtins provided by int.nl, and `string_test.nl' tests many of the predicates exported by string.nl. `interpreter.nl' is a Pure Prolog interpreter (moved from the compiler directory).
