wasm: implement a growable heap

On WebAssembly it is possible to grow the heap with the memory.grow
instruction. This commit implements this feature and with that also
removes the -heap-size flag that was reportedly broken (I haven't
verified that). This should make it easier to use TinyGo for
WebAssembly, where there was no good reason to use a fixed heap size.

This commit has no effect on baremetal targets with optimizations
enabled.

src/runtime/runtime_nintendoswitch.go

@@ -217,6 +217,13 @@ func setupHeap() {
 	}
 }
 
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	// Growing the heap is unimplemented.
+	return false
+}
+
 // getHeapBase returns the start address of the heap
 // this is externally linked by gonx
 func getHeapBase() uintptr {