Allow a default enumerator member in type definition

[joente]

Description

Allow a type definition to set a default enumerator value.

For example:

set_type('T', {
  e: 'E{M}`,
});

Where T is a new type, E the enumerator and M a member which we want as the default when not provided.

Implements issue #387.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Added enumerator tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes