inline tokenizer, take two

[mbostock]

Re-applies #1425, but without trying to change Markdown tokenization. Inline expressions must still be within blocks, but we at least now parse them correctly within the block.

Fixes #375.
Fixes #396 (by considering it intended).
Fixes #636.

[mbostock]

So this still doesn’t work (by design, because Markdown):

${html`<div>
  ${[1, 2, 3].map((i) => i)}
</div>`}

But you can easily fix it by putting it inside an HTML block:

<div>
  ${html`<div>
    ${[1, 2, 3].map((i) => i)}
  </div>`}
</div>