This patch attempts to lay some groundwork for enhancements to the Wi-Fi module
by bringing the representation of transmit rates more in line with the
standard.
The key part of the patch is the introduction of a type that corresponds to the
notion of Modulation Class described in IEEE 802.11-2007, Section 9.6.1, Table
9-2.
It also adds coding rate information to the WifiModes and centralises their
construction into a single WifiModeFactory method. The rates are also renamed
with reference to their Modulation Class.
WifiModes no longer have a WifiStandard, but the latter type still exists and is
used to imply the set of WifiModes that a MAC/PHY pair will support.
