lib: add ability to decode from lua scripts

lib: add ability to decode from lua scripts

This implements the ability to get results out from lua scripts after
they've run.

For each C type we support passing to Lua, there is a corresponding
`struct frrscript_codec`. This struct contains a typename field - just a
string identifying the type - and two function pointers. The first
function pointer, encode, takes a lua_State and a pointer to the C value
and pushes some corresponding Lua representation onto the stack. The
second, decode, assumes there is some Lua value on the stack and decodes
it into the corresponding C value.

Each supported type's `struct frrscript_codec` is registered with the
scripting stuff in the library, which creates a mapping between the type
name (string) and the `struct frrscript_codec`. When calling a script,
you specify arguments by passing an array of `struct frrscript_env`.
Each of these structs has a void *, a type name, and a desired binding
name. The type names are used to look up the appropriate function to
encode the pointed-at value onto the Lua stack, then bind the pushed
value to the provided binding name, so that the converted value is
accessible by that name within the script.

Results work in a similar way. After a script runs, call
frrscript_get_result() with the script and a `struct frrscript_env`.
The typename and name fields are used to fetch the Lua value from the
script's environment and use the registered decoder for the typename to
convert the Lua value back into a C value, which is returned from the
function. The caller is responsible for freeing these.

frrscript_call()'s macro foo has been stripped, as the underlying
function now takes fixed arrays. varargs have awful performance
characteristics, they're hard to read, and structs are more defined than
an order sensitive list.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>