Rename & refactor ByteBuf -> SliceBuf

src/alloc/heap.rs

src/alloc/mod.rs

src/imp/alloc.rs

src/imp/buf/append.rs

src/imp/buf/byte.rs

src/imp/buf/mod.rs

src/imp/buf/slice_buf.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.rs

test/test_buf_fill.rs

test/test_bytes.rs

test/test_seq.rs

test/test_slice_buf.rs

test/test_small.rs