Makefile: install headers

now you can for example
in your program and link with -lulz.

this change doesnt mean that the API will remain compatible
forever though.
add stdio-repl.h

intended to provide a libulz replacement for common stdio output
tasks by simply including the header (after stdio.h).

that way small tools that dont use a lot of libc can decrease their
static link size drastically.

OTOH, if you need a lot of other libc functions, chances are that your
binary gets slightly *bigger*, if some of those functions use
snprintf and buffered io (using FILE*) internally already.