Improve naming of types and modifiers

[soc]

internal -> intrinsic

I believe it might be a more precise name and frees up internal as a possible visibility modifier, if we ever need it.

Str -> String

It's more consistent with the toString function, and in general avoiding abbreviations makes things slightly more approachable as there are often multiple hypothetical abbreviations, but usually only one way to write the full name.

[soc]

Please note that this branch is currently sitting on the filename branch, so it would make sense to review the PR about filename first, and then I'll rebase this branch onto master.