vm: implement slot-related opcodes

1. Slot is a new mechanism for storing variables during execution
which is more convenient than alt.stack. This commit implements
support for slot opcodes in both vm and compiler.
2. Remove old alt.stack opcodes.
3. Do not process globals at the start of every function, but instead
load them single time at main.

pkg/compiler/analysis.go

@@ -1,11 +1,14 @@
 package compiler
 
 import (
+	"errors"
 	"fmt"
 	"go/ast"
 	"go/constant"
 	"go/types"
 
+	"github.com/nspcc-dev/neo-go/pkg/vm/emit"
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 	"golang.org/x/tools/go/loader"
 )
 
@@ -47,6 +50,24 @@ func typeAndValueForField(fld *types.Var) (types.TypeAndValue, error) {
 	return types.TypeAndValue{}, nil
 }
 
+// newGlobal creates new global variable.
+func (c *codegen) newGlobal(name string) {
+	c.globals[name] = len(c.globals)
+}
+
+// traverseGlobals visits and initializes global variables.
+func (c *codegen) traverseGlobals(f ast.Node) {
+	n := countGlobals(f)
+	if n != 0 {
+		if n > 255 {
+			c.prog.BinWriter.Err = errors.New("too many global variables")
+			return
+		}
+		emit.Instruction(c.prog.BinWriter, opcode.INITSSLOT, []byte{byte(n)})
+	}
+	c.convertGlobals(f)
+}
+
 // countGlobals counts the global variables in the program to add
 // them with the stack size of the function.
 func countGlobals(f ast.Node) (i int64) {