Support set types

[rebcabin]

The following generates a good error message, but the lack of set types is a severe limiitation.

INTEGER_KINDS   : set[i32] = {1, 2, 4, 8}
LOGICAL_KINDS   : set[i32] = {1, 2, 4}
REAL_KINDS      : set[i32] = {4, 8}
COMPLEX_KINDS   : set[i32] = {4, 8}
CHARACTER_KINDS : set[i32] = {1}

/Users/brian/CLionProjects/lpython/src/bin/python /Users/brian/CLionProjects/lpython/lasr/LP-pycharm/lasr_parser.py 
code generation error: Variable type not supported 6
  --> /Users/brian/CLionProjects/lpython/lasr/LP-pycharm/lasr_parser.py:58:1
   |
58 | CHARACTER_KINDS : set[i32] = {1}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

The workaround is undesirable (that's what "severe" means):

INTEGER_KINDS   : dict[i32, i32] = {1: 1, 2: 2, 4: 4, 8: 8}
LOGICAL_KINDS   : dict[i32, i32] = {1: 1, 2: 2, 4: 4}
REAL_KINDS      : dict[i32, i32] = {4: 4, 8: 8}
COMPLEX_KINDS   : dict[i32, i32] = {4: 4, 8: 8}
CHARACTER_KINDS : dict[i32, i32] = {1: 1}

The workaround hints at a possible easy fix.