Add buf_sets() function.

It is the way to write a string, any string, e.g. "Hello", to Buf's
data. See main.c for a real example.

The implementation of buf_set() and buf_sets is copied from Go's
src/cmd/dist/buf.c:bwrite() and src/cmd/dist/buf.c:bwritestr().

I renamed to buf_set() and buf_sets() since I think that is more
clearer.

That naming was inspired by libgit2's src/buffer.h:git_buf_sets() and
include/git2/buffer.h:git_buf_set().

Signed-off-by: Thiago Farina <tfarina@chromium.org>