Linq mutator mutates Count to Sum

[richardwerkman]

Describe the bug
Count is being mutated into Sum, but this results in invalid mutations most of the time.

Expected behavior
This is only a valid mutation in the case of an enumerable of numbers. Otherwise, this will always result in invalid muations. Maybe we can think of another mutation that does make sense in more cases. Or remove this mutation.

Context
This mutation was added in the original Linq mutator implementation #185

[dupdob]

this may be addressed thanks to semantic context, which could identify when this is a numerical enumeration