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/config.go

@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"path/filepath"
 	"regexp"
-	"strconv"
 	"strings"
 
 	"github.com/tinygo-org/tinygo/goenv"
@@ -203,12 +202,6 @@ func (c *Config) LDFlags() []string {
 		ldflags = append(ldflags, strings.Replace(flag, "{root}", root, -1))
 	}
 	ldflags = append(ldflags, "-L", root)
-	if c.Target.GOARCH == "wasm" {
-		// Round heap size to next multiple of 65536 (the WebAssembly page
-		// size).
-		heapSize := (c.Options.HeapSize + (65536 - 1)) &^ (65536 - 1)
-		ldflags = append(ldflags, "--initial-memory="+strconv.FormatInt(heapSize, 10))
-	}
 	if c.Target.LinkerScript != "" {
 		ldflags = append(ldflags, "-T", c.Target.LinkerScript)
 	}

compileopts/options.go

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