Remove casting functions between the adjacent types

[PaulRBerg]

While reviewing #212, I realized that maintaining casting functions between the adjacent types and all other types is:

1. Overkill and not helpful; I don't think users need this.
2. Increasing maintenance costs in an exponential way; the more adjacent types there are, the crazier the interdependencies become, and the more 'plumbing' code has to be written.

Thus, the task is to remove all casting functions between the adjacent types and all other types, e.g. between SD1x18 and UD2x18.

Note that the casting between (i) adjacent types and SD59x18 and UD60x18 and (ii) between SD59x18 and UD60x18 should continue to be maintained given that these are the core types implementing the mathematical logic.