Compress a file, using zlib (DEFLATE).

Usage:
  v compress <type> <in> <out>

The input file is read in full, compressed, and written to <out>. If
<out> exists already, it is overwritten.

`zlib` is currently the only supported compression type:

  v compress zlib data.txt data.txt.z

On any problem (unsupported type name, unreadable input, compression
or output write error), the command prints the error and exits with a
non-zero status. This command only compresses; to decompress the
result, use another tool, or the `compress.zlib` module of a V program.
