Bring master up to date

See merge request pesco/pdf!12

Merge branch '2020-04-27_RELEASE' of gitlab.special-circumstanc.es:pesco/pdf into 2020-04-27_RELEASE

This reverts commit 1d031337.

The above commit changed string parsing to an explicit recursion. But as long
as we're using packrat (i.e. recursive descent), this is going to run us out
of stack on very long strings (viz. ones that are never closed).