Obtain name from SchemaObject; otherwise, keep existing default subtype_{idx}

[dpezely]

Where existing behavior renders field names as subtype_0, this PR extracts a name from the relevant SchemaObject, when available.

• Preserves existing behavior for fall-through cases
• This scratches an itch from processing W3C's MNX current draft mnx-schema.json
  • e.g., had 60+ instances of subtype_0 through _4 in the resulting .rs file
  • That's as of their commit 0eda842 (latest as of creating this PR)
• Existing tests pass

[ahl]

Can you add a test that demonstrates this in action?

[dpezely]

Short version: a minimal repo with a canary test that exercises this PR is mnx-canary.

Long version:

• my JSON schema fu is weak
• I have some experience modifying existing JSON schema but haven't created one from scratch prior to an attempt at writing a test case for this PR
• The would-be test's schema.json started as a heavily trimmed version of the W3C mnx-schema.json but was still too verbose
• potential schema for test
• However, exercising that schema for purpose of this PR while being idomatic with existing Typify tests would require more time than I have at the moment
• When time permits, I'll continue that effort but providing what I have now-- for keeping my response somewhat timely