The Ripple protocol represent transfer rates and trust line
qualities as fractions of one billion. For example, a transfer
rate of 1% is represented as 1010000000.
Previously, such rates where represented either as std::uint32_t
or std::uint64_t. Other, nominally related types, also used an
integral representation and could be unintentionally substituted.
The new Rate class addresses this by providing a simple, type
safe alternative which also helps make the code self-documenting
since arithmetic operations now can be clearly understood to
involve the scaling of an amount by a rate.
