Exports of a module instance are accessed through a pointer to the
instance and a function that returns a pointer to the thing being
exported form the corresponding instance. This change enables each
module instance to have their own exports.

* Remove name mangling of types and WASM_RT_ADD_PREFIX, instead all exported symbols have mangled module name as their prefix
* Add modname_import_t which contains all imported globals, memories and tables
* Add pointers to module instances of every module that the current module import a function from
* Add a pointer to module intance to wasm_rt_elem_t
* Pass corresponding module instance pointer to imported functions: pointer to where the funcion is imported from for CallExpr and pointer from wasm_rt_elem_t for CallIndirectExpr
* init_func_type() is moved to init_module() which needs to be called before any instance of the module is used

* modname_init() takes pointers to each module instance that the module imports from and set up imported globals, memories and tables correspondently
* Memories, tables and globals of spectest are moved into spectest_module_instance. Functions and exports of spectest module is set up to perform the same way as wasm2c output modules
* Only one declaration is written to output header file if the same field form the same module is imported multiple times

…ame_instance_t*)

modname_free(modname_instance_t*) frees all non-imported tables and memories in the given module_instance

This makes sure it's not possible to initialize a module while
leaving the runtime (in practice, the signal handler) uninitialized.

Restores current versions of all non-SIMD core tests and
multi-memory tests, except for two that are failing:
ref_func.wast and unreached-valid.wast

These two tests produce similar errors when run under
run-interp-spec, so will require further debugging.

To pass the ref_func.wasm and unreached-valid.wast tests,
wasm2c needs to be willing to continue translating a module
that has failed to validate. (The interpreter seems to handle
this similarly.)

Expose this behavior in wasm2c behind a
`--unsafe-ignore-validation-failure` flag, and add the final two
missing tests.