Support destructing exists with intro patterns

The syntax re-uses the existing support for pure names, namely the % and
%H patterns. Using "[% H]" is like `iDestruct ... as (?) "H"` and using
"[%x H]" (with the string-ident plugin) is like `iDestruct ... as (x)
"H"`.

This changes how these patterns are parsed. Previously, both would have
been handled as conjunctions (using IntoAnd or IntoSep, depending on
whether the hypothesis is persistent or not). This means it was possible
for the user to use "[% H]" to destruct a pure hypothesis ⌜φ ∧ ψ⌝ and
put only the first conjunct in the Gallina context, leaving the other
one in the IPM; such patterns will now break, since iExistDestruct
does not handle this use case.