Marshal.load: do not call the proc until strings have their encoding

	Ref: https://bugs.ruby-lang.org/issues/18141
	---
	 marshal.c                             |  7 +++-
	 spec/ruby/core/marshal/shared/load.rb | 62 +++++++++++++++++++++++------------
	 test/ruby/test_marshal.rb             | 17 ++++++++++
	 3 files changed, 64 insertions(+), 22 deletions(-)

	marshal.c: don't call the proc with partially initialized objects.
	 (#4866)

	For cyclic objects, it requires to keep a st_table of the partially
	initialized objects.
	---
	 marshal.c                             | 75 ++++++++++++++++++++---------------
	 spec/ruby/core/marshal/shared/load.rb | 75 ++++++++++++++++++++---------------
	 test/ruby/test_marshal.rb             | 12 ++++++
	 3 files changed, 97 insertions(+), 65 deletions(-)

[Feature #8948] [Feature #16377]

Since Regexp literals always reference the same instance,
allowing to mutate them can lead to state leak.

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.