Reorganize crate

src/alloc/mod.rs

src/imp/buf/append.rs

src/imp/buf/block.rs

src/imp/buf/byte.rs

src/imp/buf/mod.rs

src/imp/buf/ring.rs

src/imp/buf/take.rs

src/imp/bytes/mod.rs

src/imp/bytes/rope.rs

src/imp/bytes/seq.rs

src/imp/bytes/small.rs

src/imp/mod.rs

src/lib.rs

test/test_append.rs

test/test_block.rs

test/test_buf.rs

test/test_buf_fill.rs

test/test_byte_buf.rs

test/test_ring.rs