Even more features and fixes

[ZedThree]

New features:

• F2008 block construct
• Implied-do loops (from @stadelmanma 's support-implied-do-loops branch, added tests and checked it works and doesn't conflict with complex literals)
• types in array constructors: [real(real64) :: 1, 2, 3]
• renames in use statements without only: use module, foo => bar (I didn't even know this was legal!)
• F2018 select rank and assumed rank (dimension(..))

Fixes:

• no whitespace before :: in variable declarations: integer::x
• access specifier (pubic/private) in contains section of derived type definitions
• close statements with unit specifier and keyword arguments
• trailing whitespace after line continuations
• comments after line continuations

Plus tests for all of the above

Closes #3 Closes #47 Closes #73

[ZedThree]

Comments after line continuations are still a bit broken as it doesn't handle the & at the start of the next line:

    write(*, "('Testing line continuation')", &
       &  advance='no', &  ! Comment after line continuation
       &  iostat=istat)

gives:

  (write_statement
      (unit_identifier)
      (format_identifier (string_literal))
      (keyword_argument (identifier) (string_literal))
-      (ERROR
-        (comment)
-        (UNEXPECTED '&')))
+     (comment)
+     (keyword_argument (identifier) (identifier)))