Update IO instance methods

[sampersand]

Phew! This one took awhile :-P. This PR updates IO's instance methods.

More specifically, this makes the following general changes:

• Deprecates ::_ReaderPartial (renamed to IO::_Readpartial) and ::_Rewindable (now unused)
• Deprecates String::_EncodeFallbackAref (moved within to Encoding, as it's used for encoding purposes).
• Fixes a minor bug in lib/rbs/unit_test/spy.rb where .new() is called instead of .allocate(). This prevents testing instance methods named :initialize.
• Removed test/stdlib/test_helper.rb:ArefFromStringToString, as it's no longer needed
• Added in TestHelper::RUBY_EXE, used when spawning shells (IO needs it for IO#pid)
• Added test/stdlib/util/small-file.txt as a small file that can be read from fro testing
• Redid all the tests in IO_test.rb to accomodate the with_xxx forms. However, wait_writable and wait_prioroity currently don't have any tests associated with them, as I couldn't figure out a good way to test them. (However, their signatures are pretty simple)

It also makes the following changes to io.rbs:

• The %a{ruby:since:3.2.0} annotation has been added to methods introduced in ruby 3.2.0
• Moved all of IO's constant definitions inside of it
• Removed constant definitions that IO inherits from File::Constants (APPEND, FNM_CASEFOLD, LOCK_SH, etc.)
• Added IO::EWOULDBLOCKWait{Readable,Writable} constants
• IO#initialize: mode is now open_mode; updated all the options to be the entire list of accepted options (along with the **untyped that it for some reason accepts)
• IO#advise: Added IO::advice type alias, offset and len are now int?
• IO#{lineno,autoclose,sync}=: Now returns their arguments
• IO#each: Now its own function; each_line aliases it. Also added in the limit-only variant.
• IO#each_{byte,char,codepoint}: Reordered branches to have Enumerator first
• IO#external_encoding: Added in. (weird that it didnt exist, since internal_encoding existed before...)
• IO#{fcntl,ioctl}: Now uses implicits, argument also accepts bool | nil as well.
• IO#{f{data,}sync,rewind}: Always returns 0
• IO#pid: Can now return nil
• IO#pos=: Now takes int
• IO#p{read,write}: Added in
• IO#{print,puts}: objects are now _ToS not untyped
• IO#printf: format string is now string
• IO#putc: Separated cases out so return values aren't unioned
• IO#{gets,read}: reordered branches slightly
• IO#readline{,s}: Added in limit-only variant
• IO#readpartial: outbuf is now nilable
• IO#reopen: Changed (IO) -> IO to be (io) -> sef; added in optional keywords to second case
• IO#seek: amount is now nil; also added in IO::whence type alias.
• IO#set_encoding: Added in optional encoding arguments
• IO#sysread: added implicit conversions; outbuf is optional and nilable.
• IO#sysseek: ADded implicit int for amount; uses IO::whence.
• IO#timeout=: Now returns self not void (which is weird, since almost every other setter in ruby returns its argument...)
• IO#to_io: Now marked %a{pure}
• IO#to_path: Added in
• IO#ungetbyte: Argument accepts implicit string and is nilable.
• IO#ungetc: Argument can now be Integer.
• IO#write_nonblock: Can no longer return nil now

Since Ruby 3.2 moved the wait_* methods into io directly (no need for require 'io/wait'), the methods IO#wait{,_{readable,writable,priority}} have been moved from io/wait.rbs, along with IO::wait_mode and marked as `%a{ruby:since:3.2.0}.

[ParadoxV5]

• The %a{ruby:since:3.2.0} annotation has been added to methods introduced in ruby 3.2.0

We have Ruby 3.3 now (:

• #1674