[CHANGES]

• Permit infix operators to consume fewer than all the available
  symbols; allowing for RHS operands that begin with symbols without
  intervening whitespace
  
• Improved unit-testing of infix operator parser precedence issues
  
• Added XPI_OPERAND_ARITH; renumbered XPI_OPERAND_TERM to be
  non-zero. Currently zero is accepted for back-compat