Carve two modules out of fact_table_gen.m.

compiler/fact_table_check.m:
    New module containing the code that checks the argument types
    and modes of the fact table predicate.

compiler/fact_table_compile.m:
    New module containing the code that checks the facts in the
    fact table against the argument types and modes, and generates
    the fact table's .c file.

compiler/fact_table_gen.m:
    The remaining part of the old fact_table_gen.m, which contains
    the code to generate the foreign_procs implementing each
    procedure of the fact table predicate, mostly building on the C code
    put into the fact table's .c file by fact_table_compile.m.
    
compiler/ll_backend.m:
compiler/notes/compiler_design.html:
    Include and document the new modules.

compiler/add_pragma_impl.m:
    Conform to the changes above.