Enforce suitable literal number operands to be unsigned integers.

Enforce suitable literal number operands to be unsigned integers.

Except for OpConstant and OpSpecConstant, all other literal number
operands are indeed unsigned integers. So,

* Rename all *LITERAL_NUMBER* operand types to *LITERAL_INTEGER*.
* Expect unsigned integers for *LITERAL_INTEGER* operands.
* Keep MULITPLE_WORD_LITERAL untouched since it is only used by
  OpConstant and OpSpecConstant.

And we want to provide the capability to specify floating-point
numbers after !<integer> in the alternate parsing mode. So,
OPTIONAL_LITERAL_NUMBER is reserved for OPTIONAL_CIV.