Adds documentation for various things that I understand.
Adds #[allow(missing_doc)] for lots of things that I don't understand.

This only changes the directory names; it does not change the "real"
metadata names.

This is now 100% consistent. Whoops!

Thanks @catamorphism!

Let's actually give a top-level description of what's in here, eh?

Had a conversation with @cmr in IRC about some places that these
docs were confusing. The functions that advance the stream now say so.

In addition, I think that calling out the similarities to familliar C
functions should help people coming from other languages.

Added docs for stdout, stderr, print, and println.

Also fix up all the fallout elsewhere throughout core. It's really nice being
able to have the prelude.

When, occasionally, trying to read directory instead of file, `fread()`
returns `EISDIR` error. And, considering, absence of error handling,
`read_whole_stream()` just loops indefinitely.

I filled out better descriptions for all of the neccesary
functions.

Specifically, `lines` and `each_line` will not emit a trailing empty string
when given "...\n". Also, add `read_lines`, which just collects all of
`each_line` into a vector, and `split_*_no_trailing` which will is the
generalised version of `lines`.