This PR improves support for fields wider than 64 bits. This is done mostly
by fixing bugs and some refactors.

The assumption for fields wider than 64 bits is that byte order of them is
network byte order, which means that byte order is preserved during parser
or deparser.

To achieve initial support for wide fields, the following changes has been made:

- Fixed byte order of bytes emitted in deparser. Parser had proper
  implementation of this.
- Allow to assign one field to another.
- Improved code for generating table key during lookup. `exact`, `lpm` or
  `ternary` match type can used.
- Improved generation of table const entries (now mostly using Visitors).
- Improved prefix length calculation for LPM table.

The current PR allows to parse, deparse, match in tables and assign wide
fields. Subsequent PR(s) will extend wide fields support for externs.

This commit adds support for fields wider than 64 bits in implemented
checksum algorithms.