Add term_of_int and int_of_term to ratSyntax

Re-indent ratSyntax.sig

Remove TABs from ratSyntax.sig

Make ratSyntax export rat : hol_type

Not sure why this might have been commented out in the initial commit
from Jens (cda6aefc).

Add {is,mk,dest}_{literal,rat_of_num} to ratSyntax

Literals are
- injections from :num; or
- negated injections from :num; or
- one of the above divided by an injection from :num

The rat_of_num constant is the injection from :num, and hides behind the
q suffix on numerals.

extra semicolons removed from (some) .sig files

Initial version of theory and libraries of rational numbers added.