Enable nested tags of the same kind

Previously, the processing rules do not allow nesting an element inside
  of itself: ‹ ☞︎nested ☞︎tags☜︎ like ☞︎this☜︎☜︎ › do not work as
  expected.
This design choice was to (at least) appropriately handle
  ‹ ☞︎⟨this☜︎ weird ⟨case☜︎⟩ ›, where each inline should stop at the
  first delimiter.
Correct processing starts by looking for end sigils first, not start
  sigils, backtracking to the last start sigil which precedes it,
  wrapping that text, and then reprocessing the entire set of nodes
  until no more end sigils with matching start sigils are found.

This commit implements that behaviour, which is of course a fair bit
  more complicated but should improve the results.
It also changes specifying characters by Unicode codepoint to use
  curly braces rather than angle brackets, as the latter conflicted
  with the angle brackets used in links.
Specifying by Unicode codepoint still isn¦t supported in links, but
  the behaviour should be less surprising.

The old Unicode codepoint behaviour was probably broken also, in the
  case where the codepoint was not the first character in a paragraph,
  but it is fixed now.