extend import statement node with macro tag

Road to handling 'import macro ...' statement.
add parsing of 'import macro ...' statement
import macro defined by 'import macro ...'
add imported symbols iterator to translator

Road to fixing error on defining aliased imported symbol as macro.
fix bug with macro'ing aliased imported symbol

Bug example:

    -- file:macros.shn
    function fn(ctx, expr) return ctx.op"hello" end

    --- file:error.shn
    import my_macro = fn from "macros"
    macro my_macro = my_macro

$ shine error.shn
Error: [string "shine.lang.translator"]:0: imported macro body for 'my_macro' cannot be resolved
stack traceback:
  <...>
document 'import macro ...' syntax
Merge pull request #64 from leonidborisenko/import-macro-statement

Import macro with shorthand 'import macro ...' syntax