Improve performance of Combinational execute

[mkorbel1]

Description & Motivation

Improve the performance of Combinational's _execute function so that it doesn't glitch extra x's throughout the design unnecessarily. For some designs this can provide a very meaningful performance boost to simulation speed.

Related Issue(s)

Fix #106

Testing

Existing tests cover this.

Performance testing doesn't exist and would require additional work.

Backwards-compatibility

Is this a breaking change that will not be backwards-compatible? If yes, how so?

No

Documentation

Does the change require any updates to documentation? If so, where? Are they included?

No

[mkorbel1]

This is a good opportunity to add a microbenchmark