This commit adds a `capacity` field to shapes, and adds shape
transitions whenever an object's capacity changes. Objects which are
allocated out of a bigger size pool will also make a transition from the
root shape to the shape with the correct capacity for their size pool
when they are allocated.

This commit will allow us to remove numiv from objects completely, and
will also mean we can guarantee that if two objects share shapes, their
IVs are in the same positions (an embedded and extended object cannot
share shapes). This will enable us to implement ivar sets in YJIT using
object shapes.

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>

    Moves Expect library doc into io.c.
    Changes certain links to local sections, now pointing to sections in doc/io_streams.rdoc.
    Removes local sections now superseded by sections in doc/io_streams.rdoc.

getenv is a very basic function that has been in stdlib.h since
ISO/IEC 9899:1990.  There is absolutely zero need for us to redeclare.

pty.c already includes stdlib.h out of the box so we need nothing.

icc now seems to provide libutil.so that is not related to pty.
This extconf.rb wrongly finds it and adds `-lutil`, but `ruby -rpty`
fails because it cannot find libutil.so on the runtime.

http://rubyci.s3.amazonaws.com/icc-x64/ruby-master/log/20220815T210005Z.fail.html.gz
```
Exception raised:
<#<LoadError: libutil.so: cannot open shared object file: No such file or directory - /home/chkbuild/chkbuild/tmp/build/20220815T210005Z/ruby/.ext/x86_64-linux/pty.so>>
```

This change makes extconf.rb check libutil only on OpenBSD.

[Misc #18891]

Passing the optional env hash to PTY.spawn has been supported for years, but it's never documented.
More info: https://bugs.ruby-lang.org/issues/12312

IA64 support was dropped in ticket #15894, so we can drop support for
HP-UX.