Now 'length' or 'len' refers to the number of cells in an
     array. 'size' refers to the number of bytes. We used to mix them
     and it made things confusing.

It was just wrong and it just never came up for some reason. Added
     some tests to detect in the future.

The reason for the shift is to optimize for the hot-path. Most of the
     calldata read positions follow the same pattern: 'n * 32 +
     4' (the first four bytes in calldata in Solidity represent a
     function selector). Thus, most of the calldata reads are
     /unaligned/ in regards to bit packing. In order to align them, we
     perform the 12-byte shift.

remove usage of comparisons and inefficient bit manipulations