Error when serializing patterns without ktable on Lua 5.1

[jdesgats]

Steps to reproduce

> L=require'lpeg'
> Ls=require'lpeg.serialize'
> p=L.P'a'
> Ls.save(p)
stdin:1: cannot serialize C function
stack traceback:
    [C]: in function 'save'
    stdin:1: in main chunk
    [C]: ?
> =debug.getfenv(p) == package
true