New version of ebfe's patch:
 - ENTER/LEAVE_HASHLIB:
   * don't touch GIL in ENTER_HASHLIB (it's useless)
   * add mandatory argument (explicit use of "self")
 - EVP_hash():
   * restore Py_SAFE_DOWNCAST
   * simplify the code: always use the while() instead of if+while
   * use "while (0 < len)" to skip zero or negative value (even if 
pitrou told me that len should not be negative)
 - EVP_dealloc(): free the lock before the context
 - release the GIL for all calls to EVP_hash()
 - use the context lock in EVP_copy() and EVP_get_digest_size() to 
protect self
 - don't use the context lock in EVP_repr() (useless because we don't 
read OpenSSL context)
 - fix the indentation of the code (replace tab by spaces)

Some rules for ENTER/LEAVE_HASHLIB:
 * it is only needed to protect the context attribute (eg. name 
doesn't need to be protected by the lock)
 * it doesn't touch the GIL: use an explicit call to 
Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS

About the GIL:
 * EVP_DigestInit() and EVP_MD_CTX_copy() are consired fast enough to 
no release the GIL
 * The GIL is released for the slowest function: EVP_DigestUpdate() 
(called in our EVP_hash() function)