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/baremetal.go

@@ -29,6 +29,13 @@ var (
 	stackTop     = uintptr(unsafe.Pointer(&stackTopSymbol))
 )
 
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	// On baremetal, there is no way the heap can be grown.
+	return false
+}
+
 //export malloc
 func libc_malloc(size uintptr) unsafe.Pointer {
 	return alloc(size)