Make :exists and :delete up to spec

The :exists and :delete adverbial pairs on hash accesses were always
interpreted as "true".  Which makes constructs as :!exists or :delete($really)
not work.
Volunteer for #66
[spectest.data] mark t/spec/S32-str/substr.t as # icu
[src/core] add .Set and .Bag to List and Parcel
Merge pull request #149 from lizmat/adverbialexistsdelete

Make :exists and :delete up to spec
Allow slice with :exists adverb on hashes

For some reason a "die" was here, whereas the spec says it should return a
Parcel of Bool.
Added slurp(IO::Handle... support
added S32-io/spurt.t
Moved spurt into methods in IO::Handle
Merge pull request #157 from dagurval/IO-spurt

Added spurt(IO::Handle... support