Support a user context void * pointer in jerry_context_t (#1717) (#1727)

This modification makes it possible to initialize a context in such a
way that a `void *` pointer is stored inside the context and is made
available via a new `jerry_get_user_context()` API.

The pointer is initialized via a new `jerry_init_with_user_context()`
API, which calls the existing `jerry_init()`, after which it sets the
value of the new `user_context` element in the `jerry_context_t`
structure using the context allocation callback provided as the second
parameter to the new `jerry_init_with_user_context()` API. The location
of the cleanup function responsible for deallocating the pointer created
by the context allocation callback is provided as the third parameter.
This location is stored in the context along with the pointer itself.

When a context is discarded via `jerry_cleanup()`, the user context
cleanup function is called to dispose of the pointer stored within the
context.

The semantics behind the API are such that it is now possible to choose
for each context an agent which manages arbitrary user data keyed to the
given context. The agent must be chosen at context instantiation time
and cannot be changed afterwards, remaining in effect for the lifetime
of the context.

Fixes #1717

JerryScript-DCO-1.0-Signed-off-by: Gabriel Schulhof gabriel.schulhof@intel.com

jerry-core/jcontext/jcontext.h

@@ -61,6 +61,8 @@ typedef struct
   ecma_lit_storage_item_t *number_list_first_p; /**< first item of the literal number list */
   ecma_object_t *ecma_global_lex_env_p; /**< global lexical environment */
   vm_frame_ctx_t *vm_top_context_p; /**< top (current) interpreter context */
+  void *user_context_p; /**< user-provided context-specific pointer */
+  jerry_user_context_deinit_cb user_context_deinit_cb; /**< user-provided deleter for context-specific pointer */
   size_t ecma_gc_objects_number; /**< number of currently allocated objects */
   size_t ecma_gc_new_objects; /**< number of newly allocated objects since last GC session */
   size_t jmem_heap_allocated_size; /**< size of allocated regions */