Metasm update

**Important note** This needs ack/ok from @jlee-r7 / @todb-r7 to be landed!**

This pull request tries to update the current version of metasm with an up to date version from https://github.com/jjyg/metasm

* Verification

- [ ] Be sure msfconsole loads smoothly
- [ ] Try to run any exploit using Metasm, easy ones are fileformat exploits using metasm. There shouldn't be any error.

```
msf > use exploit/windows/fileformat/ca_cab 
msf exploit(ca_cab) > rexploit
[*] Reloading module...

[*] Creating 'msf.cab' file ...
[+] msf.cab stored at /Users/juan/.msf4/local/msf.cab
msf exploit(ca_cab) > 
```

- [ ] Try to use any payload/encoder using Metasm, they should work as before. Example:

```
juans-mbp:metasploit-framework juan$ ./msfpayload linux/x86/read_file PATH=/etc/passwd X > /tmp/test.elf
Created by msfpayload (http://www.metasploit.com).
Payload: linux/x86/read_file
 Length: 73
Options: {"PATH"=>"/etc/passwd"}
```

Execute the payload and/or inspect with IDA or other disassembler.