compiler: replace jump targets correctly

[fyrchik]

Currently JMP* targets are translated from relative to absolute numbers going byte-by-byte with script.
https://github.com/nspcc-dev/neo-go/blob/master/pkg/compiler/codegen.go#L1029

This is incorrect, as some bytes can be a part of other instructions and not a separate opcode.
E.g. this test fails (notice, that 0x62 == byte(opcode.JMP)):

func TestJumps(t *testing.T) {
	src := `
	package foo
	func Main() []byte {
		buf := []byte{0x62, 0x01, 0x00}
		return buf
	}
	`
	eval(t, src, []byte{0x62, 0x01, 0x00}) // will be []byte{0x62, 0x7, 0x0}
}

There are several ways to address this issue.
I propose implementing some richer intermediate representation (list of structs, not just a slice of bytes). This will also make it easier to implement byte-code level optimizations in future.