Aggregate + Clean runtime fixed point arithmetic types

[kianenigma]

Currently we have:

• Perbill
• Permill
• PerU128
• Fixed64

Need to add:

• RationalU32
• Clean up PerU128
• Make the Api and exposed functions of all of them consistent. All should expose normal, checked, saturating etc.

Maybe

• Move to a new sr-arithmetic crate or arithmetic.rs in sr-primitives.
• Extensive and unified tests for all of them.
• Should probably close Rethink fraction implementations of Staking #2558 Make use of Perbill in staking/phragmen for readibility #2908 along the way.
• There are some usages of multiplication and power of 2 in Offences reporting and slashing #3322

CC @thiolliere

[thiolliere]

instead of RationalU32 I would name it FractionalU32 because it can't exceed 1, or maybe we can make that it can exceed one.

[xlc]

Those types will be very useful to ink as well. Does it make sense to move them into a standalone fixed point rust crate?