New Foreign Function Interface (FFI)

[alastairreid]

This new interface makes the types of imported/exported
functions independent of the particular representation
choices made by the runtime system.

Instead, values of arguments and results are converted
between the type used for ASL code and a standard
representation used for C code.

The representation of types is as follows

- if the ASL type is boolean then "bool" (from stdbool.h)
- if the ASL type is string then "const char*"
- if the ASL type is integer then "int"
- if the ASL type is bits(N) then either
  - uint8_t, uint16_t, uint32_t, uint64_t if N IN {8, 16, 32, 64}
  - array uint64_t _[N'] where N' is ceiling(N / 64)
- if the ASL type is an enumeration, then that enumeration
  provided that the enumeration type has been explicitly
  exported
- if a function returns a tuple of N values, then the C function
  expects N extra arguments which should be pointers for
  the C function to write results to.

Other types cannot be used in imported/exported functions.

In addition, exported functions must not raise exceptions
and, for extra safety, we insert a runtime check that
they do not raise an exception

In addition, for each configuration variable 'v : ty', we generate
a pair of getter/setter functions

    ASL_set_config_{v}({cty} value);
    {cty} ASL_get_config_{v}();

Note that (based on the ASL semantics of configuration variables)
all changes to the configuration variable should be made
before executing any ASL code.

All the interesting stuff is in libASL/backend_c_new.ml - the rest is just changes to the runtime API to support the new FFI or to support testing.