input: Increase chunk size from 16KB to 4MB.

input: Increase chunk size from 16KB to 4MB.

This not only slightly speeds up loading files, but more importantly leads
to larger chunks being passed to input modules (and later potentially output
modules), some of which can have a large performance penalty for small chunks.

In the specific case of e.g. binary input and an .sr output file this change
can lead to a speedup of 220x or more (due to larger chunks for srzip).