Rework PerlIO documentation

Rework PerlIO documentation

- Add Layers section in the description, giving details of how layers work and encompassing the list of built-in layers
- Add various information relevant to modern usage of each layer
- Consistently refer to layers with a leading colon
- Redo :utf8 and :bytes layer descriptions
- Remove references to using the :utf8 layer for UTF-8 translation
- Add :scalar layer
- Move description of default layers that was oddly in "Querying" section to the end of "Defaults" section
- Correct default layers to specify that PERLIO=:stdio will always result in default layers of :stdio
- Update all examples to be strict-safe and check for open/binmode failure
- Capitalize references to Perl consistently