[c11] Enables type redefinitions

[ivg]

Fixes #9.

ISO/IEC 9899:1999 doesn't allow redefining a type with typedef.

But C11 standard allows redeclaration of typedef names. 6.7/3 says:

    ... a typedef name may be redefined to denote the same type as it
    currently does, provided that type is not a variably modified
    type; ...

The issue is that duplicated typedefs would generate invalid XML
documents if we will just allow them in the grammar, e.g.,

$ echo 'typedef char uint8_t; typedef char uint8_t; void foo(uint8_t);' | ctoxml
<?xml version="1.0" encoding="iso-8859-1" standalone="yes"?>
<file>
    <type id="uint8_t" store="auto">
        <char/>
    </type>
    <type id="uint8_t" store="auto">
        <char/>
    </type>
    <fundec id="foo" store="auto">
        <type>
            <void/>
        </type>
        <param name="" store="auto">
            <type ref="uint8_t"/>
        </param>
    </fundec>
</file>

To prevent this, we added an efficient deduplication procedure. This procedure is useful by itself as even before this change the generated XML could be invalid, for example, when several prototypes of the same function occur in the stream (a quite common situation).

At the end we have the following behavior (snippets are taken from our cram tests)

  $ echo 'typedef char uint8_t; typedef char uint8_t; typedef char uint8_t; void foo(uint8_t x);' |  ../printc/printc_bin.exe
  
  typedef char uint8_t;
  
  typedef char uint8_t;
  
  typedef char uint8_t;
  
  void foo(uint8_t x);


  $ echo 'typedef char uint8_t; typedef char uint8_t; typedef char uint8_t; void foo(uint8_t x);' |  ../ctoxml/ctoxml_bin.exe
  <?xml version="1.0" encoding="iso-8859-1" standalone="yes"?>
  <file>
  	<type id="uint8_t" store="auto">
  		<char/>
  	</type>
  	<fundec id="foo" store="auto">
  		<type>
  			<void/>
  		</type>
  		<param name="x" store="auto">
  			<type ref="uint8_t"/>
  		</param>
  	</fundec>
  </file>