release GIL during parsing

[Nic30]

Currently the Global Interpreter Lock is held even if only C++ code is parsing. Releasing GIL during this time will make multithreading as fast as multiprocessing without need for inter process communication.

[Thomasb81]

https://github.com/Nic30/hdlConvertor/blob/2d3458d0881483a0be7ff9a2b9628398ff211ee6/hdlConvertor/_hdlConvertor.pyx#L42

To be replace by cdef cppclass Convertor nogil:

Should do the job. (Not tested)

[Nic30]

It may not be so easy as the python AST object are build in c++, but we may just need to wrap this method in GIL.