lib+test: bufio bstream writer half (closes #18)
bstream wraps *io.stream with caller-supplied rbuf/wbuf; init,
flush, setflush, unread, isbuffered, bread, bwrite, bclose
mirror ref/hare/bufio/stream.ha modulo the bstream-vs-stream
rename (cross-module type collision, see task #21).
Default flush byte-set is "\n" (Hare flag::NONE default at
stream.ha:75). Drops the prior ww-only FLUSH_ON_WRITE flag for
Hare's flush []u8 + setflush byte-set.
bufiotest.ww adds 10 @test fns: small-write, auto-flush,
line-flush default, manual-flush, isbuffered, unread (post-read
and pre-read budgets), scanner-over-bstream unread, flush-empty
no-op, bclose flushes, setflush custom byte.
Hojun-Cho
e10e95321f