Allow unmatched connections to matched ports

[jwest115]

• Added unmatched keyword
• Updated parser to catch unmatched specifiers
• Updated connection AST and semantics to include whether the connection is unmatched. Added error handling for the case where unmatched is specified on connections that do not have matched ports.
• Updated automatic port numbering algorithm to consider unmatched connections
• Added unit tests
• Updated spec and user guide to include match constrained and unmatched concepts

Closes #249. Closes #525.

[bocchino]

Looks great! Everything was functionally correct. I made some revisions for style, clarity, and efficiency.