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.

compileopts/options.go

@@ -30,7 +30,6 @@ type Options struct {
 	LDFlags       []string
 	Tags          string
 	WasmAbi       string
-	HeapSize      int64
 	TestConfig    TestConfig
 	Programmer    string
 }