Remove prefix shared_ from all definition of the functions / types of shared pointer.

This make the name shorter, give the user full control over the prefix,
give the user full power over the name,
allow more flexibility over the kind of container (allowing switch quickly),
and uniformize with variant_, tuple_ & algo_.
This is better than the current situation and all containers will switch.

==> Break API compatibility:
To fix this quickly, just add shared_ before the name into the
macro SHARED_PTR_DEF & SHARED_OPLIST.
This will generate the same name than before.

tests/test-mshared.c

@@ -27,9 +27,9 @@
 
 #include "coverage.h"
 START_COVERAGE
-SHARED_PTR_DEF(int, int)
+SHARED_PTR_DEF(shared_int, int)
 END_COVERAGE
-SHARED_PTR_DEF(mpz, mpz_t, M_CLASSIC_OPLIST(mpz))
+SHARED_PTR_DEF(shared_mpz, mpz_t, M_CLASSIC_OPLIST(mpz))
 
 static int f(const shared_int_t p)
 {