bug#12925: 24.1; string-make-unibyte instead of string-as-unibyte

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Why not make both methods do the same: insert the bytes of the
> multibyte text into the unibyte buffer?

AFAIK it's rather unusual to need to insert a text that's multibyte into
a buffer that's unibyte.  And in those cases, the right behavior is not
always the same (sometimes it should covert using something like
locale-coding-system, sometimes it should preserve the actual
byte-sequence used internally, sometimes it should signal an error, ...).

So I think, as much as possible, we should refrain from guessing and
rather request that the coder call `encode-coding-string` or something
like that explicitly to say what they want.

> Making the buffer unibyte after insertion is a PITA, because it could
> be very slow if the text in the buffer is long.

Agreed.  In my book `set-buffer-multibyte` should signal an error if the
buffer is not empty (yes, I know it's not going to happen, but I think
it's the direction we should be headed).


        Stefan