New memory subsystem.

This is a new memory subsystem implementing heap calculations as well as new builtin functions:
 * get_memory_usage()
 * get_memory_peak_usage()
 * get_memory_limit()
It also allows to set an upper memory limit, ensuring that processed script will not be able to allocate more memory from OS.
New subsystem is based on work done in 'memory_limit' branch. Big thanks to devnexen!
This finally fixes #25.

engine/api.c

@@ -120,6 +120,36 @@ static sxi32 EngineConfig(ph7 *pEngine, sxi32 nOp, va_list ap) {
 				}
 				break;
 			}
+		case PH7_CONFIG_MEM_LIMIT: {
+				char *sMemLimit = va_arg(ap, char *);
+				if(!sMemLimit) {
+					break;
+				}
+				char *sLimitRem;
+				sxu64 nMemLimit;
+				SyStrToInt64(sMemLimit, SyStrlen(sMemLimit), (void *)&nMemLimit, &sLimitRem);
+				if(sLimitRem) {
+					switch(*sLimitRem) {
+						case 'G':
+						case 'g':
+							nMemLimit *= 1024;
+						case 'M':
+						case 'm':
+							nMemLimit *= 1024;
+						case 'K':
+						case 'k':
+							nMemLimit *= 1024;
+					}
+				}
+				if(nMemLimit >= 1048576) {
+					/* At least 1MB of heap */
+					pEngine->sAllocator.pHeap->nLimit = nMemLimit;
+				} else {
+					/* Fallback to no limit */
+					pEngine->sAllocator.pHeap->nLimit = 0;
+				}
+				break;
+			}
 		case PH7_CONFIG_ERR_ABORT:
 			/* Reserved for future use */
 			break;