Support for custom labels and left placement in equations

[mdbenito]

• Implement export of equation-lab. Export to LaTeX doesn't seem to work out of the box.
• More generally, implement support for set-binding (although this won't be seen in the stree by the converter if e.g. equation-lab is used.
• leq-number (although a custom macro equation-left using surround would render better)
• align, gather, eqsplit ?

[mdbenito]

This works for labeling eqs on the left hand side.

<assign|equation-lab-left|<\macro|body|lab>
  <\surround|<set-binding|<arg|lab>><with|mode|text|font-shape|right|(<arg|lab>)><space|5mm>|>
    <\equation*>
      <arg|body>
    </equation*>
  </surround>
</macro>>

<assign|equation-left|<\macro|body>
  <\surround|<next-equation>|>
    <equation-lab-left|<arg|body>|<the-equation>>
  </surround>
</macro>>