parse: allow dereferencing operators on all expressions

parse: allow dereferencing operators on all expressions

If it looks like a @cast, and smells like a @cast, make it a @cast!

* parse.cxx (parser::parse_dwarf_value): New step to parse dereferencing
  operands in a central place.  Add those components to existing
  target_symbol types, or else create a new @cast to contain them.
  (parser::parse_crement): Call parse_dwarf_value as the next step.
  (parser::parse_cast_op,parse_atvar_op): Split from parse_target_symbol.
  (parser::parse_value,parse_symbol): Don't worry about target_symbol
  types now, as parse_dwarf_value will do it.
  (parser::peek_target_symbol_components): Look for dereferences.
  (parser::parse_defined_op): Parse any expression operand, as below.
* staptree.h (defined_op): Take any expression as the operand.
* tapsets.cxx (var_expanding_visitor::visit_defined_op): Adapt to
  @defined's new generalized operand.