Partial trust is broken by use of Span

[slozier]

It looks like using Span breaks partial trust scenarios. While trying to enable the IronPythonTest.EngineTest.ScenarioPartialTrust I found that LiteralParser.ParseString ends up throwing a VerificationException.

It seems to me like being able to use Span is preferable to enabling the partial trust scenario (particularly since the concept was dropped in .NET Core).

Related to #896