Deleting tests since I'm not using them.
Bindings are now first replaced by :identifier tokens. Syntactic sugar is then introduced. Finally, the identifiers are replaced by their values.

This allows !bind(foo)(bar baz) *foo* to result in <em>bar baz</em>. Previously one had to type *(foo)*.
Added support for mtext.
Some improvements, as suggested by the static analyzer Kibit (https://github.com/jonase/kibit).
Now always explicitly setting lspace=0 and rspace=0 for prefix and postfix operators (respectively).
Now ussing MathML's merror for error messages. This makes it possible to nest error tokens in MathML expressions.
Added n-ary lambdas.