Drop Devel::Pragma prereq

Devel::Pragma is only used for a single trivial sub, but loads a
significant amount of code.  It also causes failures on threaded perl.
Remove it as a prereq, and just inline the small bit of code we want
from it.
Fix hints hash bugs by requiring Lexical::SealRequireHints

Devel::Pragma was being used to work around bugs with the hints hash in
old perl versions.  That code was removed from Devel::Pragma though, in
favor of using Lexical::SealRequireHints.  Load that module ourselves to
avoid the bugs, without needing Devel::Pragma itself.
Merge pull request #91 from haarg/remove-devel-pragma

Drop Devel::Pragma prereq