complete tests for type-utilities in utils package

[mr-zwets]

In the cashscript/utils package there are some long-oustanding todo-s who have been there 4 years

describe('type utilities', () => {
  describe('explicitlyCastable()', () => {

    // TODO: fully test cast combinations
  });

  describe.skip('TODO: implicitlyCastable()', () => {
  });

  describe.skip('TODO: resultingType()', () => {
  });

  describe.skip('TODO: arrayType()', () => {
  });

  describe.skip('TODO: implicitlyCastableSignature()', () => {
  });
  ...
}