Error if multiple candidates for a type are found

[adamw]

If a value is provided e.g. through both a resource and an instance, report a compile-time error

[mbore]

Resolved in #188
test: https://github.com/softwaremill/macwire/pull/188/files#diff-8b70e9443678548fb7f421475afa3fe4c67401e541bbd20672eb9df1b8de3bc4