Extend `char` support

[tmteam]

literal, comparasing, increment

# literal:
  a:char = 'a'[0]
  b:char = 'z'[0]

# operators:
  a<b #true
  'x'[0] in [a..b] # true    #Excluded. It will be solved in separate task

# functions:
    char.toText()
    byte.toChar() #Excluded. toXXX functions are not implemented
    a.increment().increment() # 'c'     #Excluded. It will be solved in separate task

# convert():
    text->char
    uint->char->uint #Same for every int type
    char -> byte[]

[tmteam]

'x'[0] in [a..b] - will be done in separate task