Refactor memory management (#2954)

This PR is a general cleanup for garbage collection and memory
allocation code paths.

Changes:
  * Removed an unnecesary local variable from 'ecma_gc_mark'.
  * Refactored 'ecma_gc_run' to have an implicit list head during
    iteration, which results in one less condition in the loops,
    and changed the loops to use compressed pointers to reduce the
    overall amount of compression/decompression.
  * Renamed 'jmem_free_unused_memory_severity_t' to 'jmem_pressure_t',
    and added additional values.
  * Removed 'jmem_free_unused_memory_callback', instead
    'ecma_free_unused_memory' is now called directly.
  * Reworked 'ecma_free_unused_memory' to handle all code paths related
    to 'jmem_pressure_t', and moved all relevant code paths into this
    function. This simplifies the code paths in other places.
  * Reworked 'jmem_heap_gc_and_alloc_block' to be more streamlined.
  * Changed mem-stats to not report unused pool chunks as allocated
    memory.
  * Created an allocator internal API for allocating/freeing memory blocks
    that are not reported as used memory in mem-stats.
  * Removed iteration statistics for the jerry allocator from mem-stats,
    as they don't provide any actually useful information.

Co-authored-by: Marko Fabo <mfabo@inf.u-szeged.hu>
JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

jerry-core/include/jerryscript-core.h

@@ -111,10 +111,10 @@ typedef enum
  */
 typedef enum
 {
-  JERRY_GC_SEVERITY_LOW, /**< free unused objects, but keep memory
+  JERRY_GC_PRESSURE_LOW, /**< free unused objects, but keep memory
                           *   allocated for performance improvements
                           *   such as property hash tables for large objects */
-  JERRY_GC_SEVERITY_HIGH /**< free as much memory as possible */
+  JERRY_GC_PRESSURE_HIGH /**< free as much memory as possible */
 } jerry_gc_mode_t;
 
 /**