microbit: Added support for MicroBitHeapAllocator to allocate C++ style array

declarations

Added missing operator new[] wrapper in MicroBitHeapAllocator.h
Ammended defualt microBitConfig.h to allocate 90% of available heap, rather
than 95%. this is to leave a little more heap for external libraries.

inc/MicroBitConfig.h

@@ -39,7 +39,7 @@
 
 // The proportion of SRAM available on the mbed heap to reserve for the micro:bit heap.
 #ifndef MICROBIT_HEAP_SIZE
-#define MICROBIT_HEAP_SIZE				0.25
+#define MICROBIT_HEAP_SIZE				0.9
 #endif
 
 // if defined, reuse the 8K of SRAM reserved for SoftDevice (Nordic's memory resident BLE stack) as heap memory.

inc/MicroBitHeapAllocator.h

@@ -91,6 +91,14 @@ inline void* operator new(size_t size) throw(std::bad_alloc)
     return microbit_malloc(size);
 }
 
+/**
+  * Overrides the 'new' operator globally, and redirects calls to the micro:bit theap allocator.
+  */
+inline void* operator new[](size_t size) throw(std::bad_alloc)
+{   
+    return microbit_malloc(size);
+}
+
 /**
   * Overrides the 'delete' operator globally, and redirects calls to the micro:bit theap allocator.
   */