Snapshots: file name collisions and formatting that hides changes; missing features #101

Review findings for the Snapshots, split into one issue per topic. The details, file:line references and verification status are in the sub-issues.

Milestone 7.x (no breaking changes)

• Snapshots: normalize line endings #118
• Snapshots: safer handling of missing snapshots #119
• Snapshots: normalizers, obsolete snapshot report, strictness options, more types #120

Milestone 8.0 (breaking changes)

• Snapshots: collision-free file names with a rename-only migration mode #115
• Snapshots: formatting that hides changes (HTML, JSON-LD, JSON numbers) #116
• Snapshots: SKIP_SNAPSHOT_TESTS should skip only the snapshot assertion #117

Removal of deprecated APIs and UPGRADE-8.0.md: #137