Add a function `OptionFromPtr` that constructs an `Option` from a
pointer. The function signature is: `OptionFromPtr : *A -> Option[A]`.
This utility has proven useful in the taproot-assets project.
In this commit, we add two new methods that simplify usage of an
handling a value wrapped in an Option. Thes methods allow a caller to
force the nil/None check up front, and either obtain the value, or fail
the current execution or test.
This should replace most usages of `UnsafeFromSome` where ever used
today.
