Signature fields named nil, true or false keep their names

[deepfates]

The property test "signature dump/load round-trips the serialized contract" failed on #247's CI with the generated spec "nil: string -> a: string": the loaded signature's input was :"" where it was nil. This happens on main and on v0.5.0 as well. The property hit it by chance.

The cause: a field name written as text becomes its existing atom, and "nil" became the atom nil. Field.dump/1 then wrote to_string(nil), which is "".

A name written as nil, true or false now keeps its text, so the field is named "nil" and saves and loads as "nil". A field given the atom nil, true or false as its name is refused with ArgumentError, since those atoms are values, not names.

Two tests in test/property_invariants_test.exs:

• fields named nil, true and false keep their names through dump, JSON and load;
• Field.new(nil, :input) raises.

With main's field.ex, both fail (5 properties, 2 tests, 2 failures). With this change: 5 properties, 35 tests, 0 failures across the property, repeated-names, signature-optimizer and doc-contract tests. mix compile --warnings-as-errors is clean.