Add an expectation that can assert the type of a value without specifying the content of the value

[popematt]

In the conformance tests, it would be useful to write test that assert that some input is or is not some type.

For example, one might want to write a test that asserts that when the null keyword is quoted that should not be interpreted as a null value. While we could assert that it is the symbol or string with the text "null", that requires having implemented support for symbols/strings already.

(ion_1_x "the null keyword must be unquoted to be a null value"
         (text " 'null' ")
         (denotes (any (not Null))) // or perhaps (any !Null)

Slightly less ideal (for test isolation), but simpler to read would be something like this:

(ion_1_x "the null keyword must be unquoted to be a null value"
         (text " 'null' ")
         (denotes (any Symbol))