Make ft_parse inputs const, move token_free

Make ft_parse inputs const, move token_free

After playing a little with the functions I thought it stupid to have
all tokens have their own memory for types that repeat so much.
The token_free function I implemented frees both members of token
which leads to multi-frees when reusing a string for type.

This is why I think it will be better to hide token_free again
and the user can decide what to allocate and what to free.

For this to be possible I have guarantee the tokens vector is not
changed inside the ft_parse function, so I've rewriten it a little
to use const.