A couple of small size optimizations

[justinfagnani]

• Use _updateComplete Promise to wait for first connection
• Optimize _enqueueUpdate for size a bit
• Inline _markUpdated
• Inline and combine style flattening and de-duplication

Before:

┌─────────────────────────────────────────┐
│                                         │
│   Destination: lit-element.bundled.js   │
│   Bundle Size:  21.04 KB                │
│   Minified and Gzipped Size:  6.75 KB   │
│   Brothli size: 6.13 KB                 │
│                                         │
└─────────────────────────────────────────┘

After:

┌─────────────────────────────────────────┐
│                                         │
│   Destination: lit-element.bundled.js   │
│   Bundle Size:  20.7 KB                 │
│   Minified and Gzipped Size:  6.65 KB   │
│   Brothli size: 6.04 KB                 │
│                                         │
└─────────────────────────────────────────┘