Compute the middle of two numbers x and y, which is equivalent in both value and
type to computing their mean ((x + y) / 2).
julia> middle(now(), now())
ERROR: MethodError: no method matching middle(::DateTime, ::DateTime)
ERROR: MethodError: no method matching middle(::Char, ::Char)
Char seems likely.
For DateTime, which has 1 ms precision, I’m not sure what the rounding rules are.
For Date, which has 1 day precision, likewise.
Are there other types that should have these methods?