When user-defined function is called, it does convert all the arguments using
MustConvert, which panics on error with an error. However 'func (e expression)
Evaluate(ctx Context)' from sub package "expressions" catches those panics and
converts some of them to errors. Raw errors returned from strconv functions
are "repanicked", but proper error types such as values.TypeError are not.
Hence we should use it here.
Go's reflect API is very picky about arguments to Call method. You can't just
pass in the "int" when function expects "int64", even on 64 bit machines. This
patch solves this problem by performing conversions properly. Truncation
problems however, as well as negative to uint issues are completely ignored.
Which is perhaps not ideal, but still better than returning "int" when "int32"
or "int64" is requested.
I did split the Kind switch cases and moved value -> int and value -> float
conversions to separate functions. An alternative to that would be using the
reflection API, which might be less performant. Not that it matters much, but
this solution is correct, even though looks a bit copy & pasty.
Oh and of course all "uint" types were completely missing in the function and
they are now added.
