compiler: move it up from vm

It really deserves it, I think. Especially given that it doesn't have any
direct usage of `vm` package now.

pkg/compiler/compiler.go

@@ -0,0 +1,100 @@
+package compiler
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/parser"
+	"go/types"
+	"io"
+	"io/ioutil"
+	"os"
+	"strings"
+
+	"golang.org/x/tools/go/loader"
+)
+
+const fileExt = "avm"
+
+// Options contains all the parameters that affect the behaviour of the compiler.
+type Options struct {
+	// The extension of the output file default set to .avm
+	Ext string
+
+	// The name of the output file.
+	Outfile string
+
+	// Debug outputs a hex encoded string of the generated bytecode.
+	Debug bool
+}
+
+type buildInfo struct {
+	initialPackage string
+	program        *loader.Program
+}
+
+// Compile compiles a Go program into bytecode that can run on the NEO virtual machine.
+func Compile(r io.Reader) ([]byte, error) {
+	conf := loader.Config{ParserMode: parser.ParseComments}
+	f, err := conf.ParseFile("", r)
+	if err != nil {
+		return nil, err
+	}
+	conf.CreateFromFiles("", f)
+
+	prog, err := conf.Load()
+	if err != nil {
+		return nil, err
+	}
+
+	ctx := &buildInfo{
+		initialPackage: f.Name.Name,
+		program:        prog,
+	}
+
+	buf, err := CodeGen(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	return buf, nil
+}
+
+type archive struct {
+	f        *ast.File
+	typeInfo *types.Info
+}
+
+// CompileAndSave will compile and save the file to disk.
+func CompileAndSave(src string, o *Options) ([]byte, error) {
+	if !strings.HasSuffix(src, ".go") {
+		return nil, fmt.Errorf("%s is not a Go file", src)
+	}
+	o.Outfile = strings.TrimSuffix(o.Outfile, fmt.Sprintf(".%s", fileExt))
+	if len(o.Outfile) == 0 {
+		o.Outfile = strings.TrimSuffix(src, ".go")
+	}
+	if len(o.Ext) == 0 {
+		o.Ext = fileExt
+	}
+	b, err := ioutil.ReadFile(src)
+	if err != nil {
+		return nil, err
+	}
+	b, err = Compile(bytes.NewReader(b))
+	if err != nil {
+		return nil, fmt.Errorf("error while trying to compile smart contract file: %v", err)
+	}
+
+	out := fmt.Sprintf("%s.%s", o.Outfile, o.Ext)
+	return b, ioutil.WriteFile(out, b, os.ModePerm)
+}
+
+func gopath() string {
+	gopath := os.Getenv("GOPATH")
+	if len(gopath) == 0 {
+		gopath = build.Default.GOPATH
+	}
+	return gopath
+}