Eliminate RenderContextInner

[cmrschwarz]

Hey, I realized that there's one last thing I think we could consider before v6.0.

Now that we no longer clone the RenderContext we could get rid of the slightly hacky RenderContextInner
and the whole writing through an Rc idiom associated with it.

I know I initially said that I don't expect any more breaking changes,
but I missed that this narrows the lifetime contract on the public RenderContext type.
This shouldn't really affect anybody ('reg: 'rc is basically assumed everwhere already),
but as a matter of technicality might be semver incompatible.

This should now really be the last breaking change in the indent patch series though :).

[coveralls]

coverage: 81.95% (+0.4%) from 81.548%
when pulling 64b709c on cmrschwarz:eliminate_rc_inner
into 080b5e0 on sunng87:master.

[sunng87]

Sounds good to me!