Also, fix regular macros so they're expanded at the
point of use, and NOT also the point of definition.
`\let` macros, by contrast, are expanded at the
point of definition. Added an `ExpansionPoint`
field to `Macro` to track this difference.
Fixed applyMacros so that it operates on the whole
string, not just the first token!
Don't remove macro definitions from the output,
even if Ext_latex_macros is set, so that macros will
be applied. Since they're only applied to math in
Markdown, removing the macros can have bad effects.
Even for math macros, keeping them should be harmless.
