Reorganize heap context related elements (#2500)

- Moved global context's `JMEM_HEAP_SIZE` to jcontext.h as external
  context's heap size is declared there, too.
- Moved the assert on `jmem_heap_t` vs `JMEM_HEAP_SIZE` to jcontext.c,
  as both entities are declared in the corresponding header.
- Removed superfluous checks on `JMEM_HEAP_SIZE` as it is not a
  publicly configurable macro (opposed to `CONFIG_MEM_HEAP_AREA_SIZE`).
- Ensured that all definitions of and references to `jmem_heap_t`,
  `JERRY_HEAP_CONTEXT`, `JERRY_HEAP_SIZE`, and `JERRY_HEAP_AREA_SIZE`
  are guarded by `#ifndef JERRY_SYSTEM_ALLOCATOR`, as they are
  meaningless if the system allocator is used.

The commit also contains some stylistic changes in jcontext.h

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/jcontext/jcontext.c

@@ -20,11 +20,20 @@
  */
 
 #ifndef JERRY_ENABLE_EXTERNAL_CONTEXT
+
 /**
  * Global context.
  */
 jerry_context_t jerry_global_context;
 
+#ifndef JERRY_SYSTEM_ALLOCATOR
+
+/**
+ * Check size of heap is corresponding to configuration
+ */
+JERRY_STATIC_ASSERT (sizeof (jmem_heap_t) <= JMEM_HEAP_SIZE,
+                     size_of_mem_heap_must_be_less_than_or_equal_to_JMEM_HEAP_SIZE);
+
 /**
  * Jerry global heap section attribute.
  */
@@ -34,11 +43,11 @@ jerry_context_t jerry_global_context;
 #define JERRY_GLOBAL_HEAP_SECTION JERRY_ATTR_SECTION (JERRY_HEAP_SECTION_ATTR)
 #endif /* !JERRY_HEAP_SECTION_ATTR */
 
-#ifndef JERRY_SYSTEM_ALLOCATOR
 /**
  * Global heap.
  */
 jmem_heap_t jerry_global_heap JERRY_ATTR_ALIGNED (JMEM_ALIGNMENT) JERRY_GLOBAL_HEAP_SECTION;
+
 #endif /* !JERRY_SYSTEM_ALLOCATOR */
 
 #endif /* !JERRY_ENABLE_EXTERNAL_CONTEXT */