Fail typechecking for functions passed to `bigslice.Func` that take `func` and channel arguments

[jcharum]

Right now, these aren't caught until we try to gob-encode. Consider failing faster in type-checking to avoid too much confusion/loss when it works with local execution.

[ogi-iii]

Can I ask more details of this issue?

I think I should fix the function "Func(fn interface{}) *FuncValue {...}" in bigslice/func.go (from line 152), but what kind of typechecking should I add this code?
Could you give me an example of failed typechecking pattern?

I would like to work on this issue as my first experience of OSS committer!