Optimise + document Table library

[OttoHatt]

Updated some of the table util functions to use the newer table. methods. Hopefully they'll be faster!(?). Also changed some doc phrasing.

I've done my best to test, but I know changes to a module like this can incur a lot of hard-to-find and obscure bugs (likely to do with mix/matching dictionary-style and array-style tables). Consider testing thoroughly with older nevermore projects, or simply not merging????

[OttoHatt]

It may be a good idea to rename the .readonly methods; they provide the nice functionality in erroring on a nil index, but I think it's easy to assume it does the same as table.freeze (which errors on write but not on nil indexing).

[Quenty]

Closed in favor of #321