Fix edge cases for symbol quoting rules

[HertzDevil]

There are three edge cases addressed here:

• :&** shouldn't require quotes when inspected, because it is a valid operator name.
• The empty symbol (:"") should require quotes when inspected, but currently produces just :, which is syntatically invalid. This affects SymbolLiteral#to_s and indirectly methods such as NamedTuple#inspect:

  # the following ultimately expands to `self[:].inspect(io)`
  p({"": 1}) # Error: expecting token ']', not ':'

• Although the operator symbols and the underscore symbol (:_) don't require quotes in the symbol notation, they do need them when used as names (e.g. named argument keys, external parameter names). For example:

  {+: 1, -: -1}           # Error: unexpected token: :
  {"+": 1, "-": -1}       # Okay
  {{ {"+": 1, "-": -1} }} # Error: unexpected token: :
  
  NamedTuple(_: String)         # Error: expecting token ')', not ':'
  NamedTuple("_": String)       # Okay
  {{ NamedTuple("_": String) }} # Error: expecting token ')', not ':'
  
  macro foo(call)
    {{ call }}
    {% debug %} # => bar(*: 1)
  end
  
  foo bar("*": 1) # Error: unterminated call

[bcardiff]

I merged master to resolve the conflicts due to recent PRs.