C representation: introduce assignment; actions subsume expressions.

 * introduce assignment, and hence lvalues.  Currently lvalues are
   variables or index expressions:

     lvalue ::= local-variable
                lvalue [ expression ]

 * Expressions can now appear wherever effectful computations can appear.
Give the optional bytecode-only stub an explicit representation.
Define C definitions as C declarations with bodies.
Formatting for C declarations.
Add support for local constants to the C representation.
Add CAMLreturnT support.
Add support for CAMLparam and CAMLlocal macros.
Make it easier to refer to local variables during C generation.
Functions for generating inverse C stubs.
Add Cstubs_generate_ml.inverse_case.
A function for generating constructors for typed function table indexes.
Add Cstubs_inverted interface.
Add Cstubs_inverted implementation.
Update Makefile with Cstubs_inverted interface.
Update dependencies for Cstubs_inverted.
Merge pull request #150 from yallop/inverted_stubs

Support for exposing OCaml code as C libraries
Clarify when copying occurs.
Mention coercion transitivity.
Merge pull request #152 from yallop/ocamldoc-updates

Documentation updates
Cstubs.write_c now includes headers that are needed in generated code.