Update optimization and porting docs to reflect global namespace cleanup

[tizoc]

When https://github.com/Shen-Language/shen-sources/pull/52 gets merged, a bunch of the stuff in the wiki will be outdated, don't forget to update:

• https://github.com/Shen-Language/wiki/wiki/System-Functions remove this one, it is part of the sources repo already. Alternatively, just make it point to the markdown file in the sources repo.
• https://github.com/Shen-Language/wiki/wiki/PortDeveloperNotes add dict.kl to the list of files to be loaded, update all info related to dicts and */or functions. Document multibyte I/O (in short, the existence of shen.read-char-code and shen.read-file-as-charlist