re-apply: Upgrade to Encode-2.60

Commit dc396cc293 was reverted by 547245703 due to issues under g++.
This commit re-applies the original commit; the next commit will fix those
issues.

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This upgrade includes the blead customizations from 080a704606 and
9e9002efd1 (amongst other changes) so that Perl 5.20.0 can ship with an
official CPAN release of Encode.
Fix Encode 2.60 with g++

The recently added Encode 2.60 won't compile with g++, due to assigning
a const char * const pointer to a char* struct field.

The intent of the code itself is a bit unclear, but it appears to be
to set SvPVX as a read-only alias of a const string, using the SvLEN()=0
trick to avoid it being freed.

Fix the g++ builds by casting away the constness, and add some asserts and
comments to make it less unclear what's going on.