Remove the various PartialEq<Value> impls for primitives

Currently we have:

- impl PartialEq\<Value> for str
- impl PartialEq\<Value> for &'a str
- impl PartialEq\<Value> for String
- impl PartialEq\<Value> for i64
- impl PartialEq\<Value> for u64
- impl PartialEq\<Value> for f64
- impl PartialEq\<Value> for bool

These lead to surprising inference errors in code that is unrelated to Serde. One example is #357.