Modules bind a single identifier 'module'
to a hash table containing keys 'ops' and 'vars'

Note: include doesn't work yet!
Basic working version - no vars, no conditions
Vars in rules, partial condition support
Remove unneeded modules
Test cases adapted to new conventions
Check number of arguments in term construction

Note: variable-length argument lists are not allowed here yet,
      but they are used liberally inside modules.rkt
Code reorganization, builtin modules, special ops
Module import, reorganization of builtin modules
Remove unneeded meta-operator
Improved meta-term parser
Operator overwrite protection
Move all module-related stuff to modules.rkt
Put internal functions of module-from-meta back in the right place
Shortened define-op to op
Reorganized unit tests
Builtin module "string"
Builtin module "symbol"
Second import mode "extend"
Check for unused vars in rules
Allow atomic terms (strings, numbers, ...) in rewriting