Change ambiguity warnings to errors and delay until called

[quinnj]

As mentioned in the Speed vs. Correctness JuliaCon presentation, it was generally thought to be a good idea to change ambiguity warnings to errors and to change them from being thrown at compile time to call-time (runtime).

Related to quite a number of open issues:

• Hit-and-miss method ambiguity detection? #6187
• Unnecessary ambiguity warning? #5460
• Method ambiguity detection: false positive? #6048
• ambiguity warning missing? #3025
• Extraneous ambiguity warning: (MyType{T},T) vs (T,MyType{T}) #1631
• confusing dispatch when method parameter bounds are wider than a type parameter's declared bounds #6383

[JeffBezanson]

dup of #6190

[quinnj]

Ah, thanks. I should have gone to the 2nd page of search results for "ambiguity" ;)

[JeffBezanson]

Yeah there are a lot of those :)

This will also fix #270, since there won't be a warning any more, and we can allow ambiguity-resolving definitions to be added at any time.

However we will also have to make ambiguity detection more accurate, since more will be at stake than just an annoying message.