Switch primitive types to be singletons

Ensure that exactly one of each primitive type can exist.

For #82
Merge pull request #85 from TysonAndre/primitive_singletons

Switch primitive types to be singletons
Resubmitting Enums pull request.
Merge pull request #88 from maxqchen/master

Resubmitting Enums pull request.
refactor: rename "endpoint" to "port"
additional renaming of Endpoint to Port
rename ept to pt
Merge pull request #90 from mtrberzi/issue72

refactor: rename "endpoint" to "port"
basic serialization
Added code coverage stats to the README
Merge pull request #100 from manifold-lang/coveralls-69

Added code coverage stats to the README
Simple check on serialization output
Merge pull request #91 from maxqchen/master

Implemented basic serialization for intermediate schematics