fix build error in libstdc++
remove 'identifier' node
remove 'procedure' node

it doesn't cause an error any more when  proc's return type is
specified.  It should be detected at semantic check.
remove grammatical branch nodes using boost::variant type aliases
remove operator symbols and use string instead

this is prepare for integrating all binary and unary expressions
check procedure's return type
fix token length when the length is zero
fix priority of parse
check build with clang-3.5

because libclang-3.5 deb package seems to have been fixed
show test log on test failure
remove individual literals and integrated them into primary_literal

symbol literal remains because it needs to distinguish from string
literal