Support arbitrary infix predicates (#87)

* Simplify parsing of predicate application

* Change lexing of identifiers:

* schematic symbols can only be variable-like: a mix of letters, digits, and _, preceded by '
* constant symbols can be:
  - variable-like
  - sequence of digits
  - sequence of selected ASCII characters to allow ids such as ++, ::
  - 1 arbitrary non-whitespace symbol
* constant symbols can be path-qualified (a path is words, separated by $: abc$def$): algebra$matrices$*
  Since paths are currently not supported by LISA, the path gets merged into the id on the lexer level.
  This is a temporary behaviour to be changed once paths are supported.

* Generalize supported infix predicates

In addition to equality, parse as infix all predicates on whose names isInfix
returns true