Error message for bad types in number operations should be improved

[floitsch]

main:
  x /any := "str"
  print 3 + x

yields

Class 'string' does not have any method 'add_from_small_integer_'.
  0: SmallInteger_.+           <sdk>/core/numbers.toit:966:20
  1: main                      bad.toit:3:11
  2: __entry__.<lambda>        <sdk>/core/entry.toit:48:20

Would be nice to have a type-mismatch error instead.

[floitsch]

Since we are intercepting operations anyway, we only reach the methods when the shortcut fails. As such we should probably just add type annotations to the methods.