Improve coverage for "spread" operator

Ref tc39/ecma262#410

The operand of the spread operator may be an AssignmentExpression. Add
test cases which demonstrate this.

Formulate templates for the SpreadElement in an array initializer in
order to conform to existing pattern.

The latest revision of ECMA262 makes special provisions for classes
which extend the `null` value [1]. Update the relevant tests
accordingly.

[1] https://github.com/tc39/ecma262/issues/543

These semantics are currently contested, but the test reflects the
intended behavior of the latest version of the specification.

Ensure that all bytes of newly-created Data Block are initialized to
zero.

Occurs that when a PutValue or GetValue operation is applied to a
property reference with a primitive base, the base is first coerced to
an Object.

Re-format destructuring assignment tests

Add tests for Annex B "function in block" semantics (procedurally generated)

Add tests for Lexical Environment management

Utilize the test generation tool to increase coverage of destructuring
assignment semantics. Previously, only destructuring assignment in the
AssignmentExpression position was tested. With this change applied, the
same tests will assert expected behavior for destructuring assignment in
`for..of` statements, as well.

A limited number of tests are applied to the `for..in` statement as
well, but due to the iteration protocol observed by that statement, many
destructuring tests are not relevant, and others cannot be automatically
generated from this format.

Update testTypedArrayConversions to require byteConversionValues

Add tests for Annex B extns to Date.prototype

- Add tests for abrupt completion handling from ToString operations
- Add tests for method property descriptors
- Remove redundant assertion

Add tests for the spread operator (procedurally generated)