update to zig 0.16.0; fix writer drain function and buffer

[em-dash]

update to zig 0.16.0 with some trivial changes.

fixed the drain function writing the whole buffer; write to index w.end instead. removed unnecessary splat == 0 and data.len < 1 checks; the latter is always false per the standard lib documentation.

the buffer was being set to a slice of len 0; set up a buffer correctly. added a flush to println to make sure it actually prints the line.