So that code using them doesn't have to spell it out constantly.

* Renamed str::split -> str::split_byte
* Renamed str::splitn -> str::splitn_byte
* Renamed str::split_func -> str::split
* Renamed str::split_char -> str::split_char
* Renamed str::split_chars_iter -> str::split_char_iter
* Added u8::is_ascii
* Fixed the behavior of str::split_str, so that it matches split_chars
  and split (i.e. ["", "XXX", "YYY", ""] == split_str(".XXX.YYY.", "."))
* Fixed str::split_byte and str::splitn_byte so that they handle
  splitting UTF-8 strings on a given UTF-8/ASCII byte and also handle ""
  as the others do

The direct motivation for this was that the monomorphizer needs to be
able to generate sane symbols for random items. The typechecker can
probably also use this in the future to provide more useful error
messages.

str::unsafe::safe_slice -> str::unsafe::slice_bytes_safe_range

Code is still somewhat kludgy because we don't have 32-bit enums.

Issue #1673

Now that core exports "option" as a synonym for option::t, search-and-
replace option::t with option.

The only place that still refers to option::t are the modules in libcore
that use option, because fixing this requires a new snapshot
(forthcoming).

Closes #1304

(Commit also includes lots of changes to remove '.'s that a git
merge messed up, or else it was monkeys.)

Does what it says on the tin.

The next commit will remove support for this syntax.

Rather, it is now a struct where properties like opts are accessed
directly, and the error-reporting methods are part of a static impl
(with the same name as the type).