Make size non-optional for Memory::new.
Make size non-optional for Memory::new.

Rationale for this is that the functions that do bounds-checking do not
function without it, and those that don't yet do bounds-checking will
at some point in the future. So, there's just no reason to allow
zero-size wrappers when they don't make sense and are error-prone as
hell.

nilium/ruby-snowdata,nilium/ruby-snowdata