Changes

[minor] printlambda: print the 'tailcall' attribute in the same style as others
 (commit: 792deb1)

[refactoring] gives tailcall attributes a more standard structure

We want to start allowing more information in the payload of
[@tailcall] attributes (currently no payload is supported), for
example we could consider using [@tailcall false] to ask the code
generator to disable a tail call.

A first required step in this direction is to use a custom datatype to
represent the tail-call attribute, instead of a boolean. This is
consistent with the other application-site
attributes (inline_attribute, specialise_attribute, local_attribute),
so it makes the code more regular -- but the change itself is
boilerplate-y.
 (commit: d260a79)