Rename operators.

[python-processing-unit]

Rename the following operators:

• ADD -> +
• SUB -> -
• MUL -> *
• DIV -> /
• FADD -> F+
• FSUB -> F-
• FMUL -> F*
• FDIV -> F/
• IADD -> I+
• ISUB -> I-
• IMUL -> I*
• IDIV -> I/
• TADD -> T+
• TSUB -> T-
• TMUL -> T*
• TDIV -> T/
• MADD -> M+
• MSUB -> M-
• MMUL -> M*
• MDIV -> M/

Update the implementation, specification, tests, and stdlib.