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/jmem/jmem.h

@@ -25,13 +25,6 @@
  * @{
  */
 
-#ifndef JERRY_ENABLE_EXTERNAL_CONTEXT
-/**
- * Size of heap
- */
-#define JMEM_HEAP_SIZE ((size_t) (CONFIG_MEM_HEAP_AREA_SIZE))
-#endif /* !JERRY_ENABLE_EXTERNAL_CONTEXT */
-
 /**
  * Logarithm of required alignment for allocated units/blocks
  */