esp8266: Change bignum from mpz to longlong; move some rodata to iram.

Some rodata items can go in iram/irom segment, but not others.  With
this patch ESP now has 24256 bytes of heap ram.  It passes 228 out of
248 tests from tests/basics directory.
diff --git a/esp8266/mpconfigport.h b/esp8266/mpconfigport.h
index 8a225a0..fbf77f6 100644
--- a/esp8266/mpconfigport.h
+++ b/esp8266/mpconfigport.h
@@ -31,7 +31,7 @@
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_STDFILES     (1)
 #define MICROPY_CPYTHON_COMPAT      (0)
-#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
+#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_LONGLONG)
 #define MICROPY_FLOAT_IMPL          (MICROPY_FLOAT_IMPL_NONE)
 #define MICROPY_ERROR_REPORTING     (MICROPY_ERROR_REPORTING_TERSE)