Use data type information from headers provided by the user

We should make it possible to pass headers to RetDec that will be used to extract information about function data types. Then these types would be applied to functions during decompilation.

Original email from user:
_I have a dll and accompanying header file that I want to decompile. I cannot see how to feed the header file into the decompilation process and without the header file, the decompiled export functions have (void) argument list, which is incorrect of course. In fact, I see that all the internal functions also have (void) argument list. This seems wrong too._

Start by reviewing how RetDec accepts decompilation inputs and how it currently derives function signatures from a DLL. Use the reported accompanying header and the incorrect void argument lists as the acceptance case; done means user-provided declarations are applied to exported and relevant internal functions during decompilation.

Written by the indexing model from the issue text.