Fixed point types

[chriseth]

https://www.pivotaltracker.com/story/show/81779716

TODO:

• assignment (lvalue and rvalue)
• conversion (between different fixed types)
• conversion (between other types)
• comparison operators (< > =)
• unary operators (-, --, ++)
• binary operators (+ - / * )
• do-we-need-these? binary operators (% **)

[chriseth]

Some notes:

IntegerType::binaryOperatorResult is too tight, this should work but seems not to: uint128(1) + ufixed(2), same with FixedPointType::binaryOperatorResult

also this should work: .5 * uint128(7)

what happens currently with uint(7) / 2? Is it identical to .5 * uint(7)?

add test about signed mod with rational constants (should behave identical to SMOD opcode)