Autodetect __int128 availability on the C side

Instead of supporting configuration of the field and scalar size independently,
both are now controlled by the availability of a 64x64->128 bit multiplication
(currently only through __int128). This is autodetected from the C code through
__SIZEOF_INT128__, but can be overridden using configure's
--with-test-override-wide-multiply, or by defining
USE_FORCE_WIDEMUL_{INT64,INT128} manually.

contrib/travis.sh

@@ -14,7 +14,7 @@ fi
 
 ./configure \
     --enable-experimental="$EXPERIMENTAL" --enable-endomorphism="$ENDOMORPHISM" \
-    --with-field="$FIELD" --with-bignum="$BIGNUM" --with-asm="$ASM" --with-scalar="$SCALAR" \
+    --with-test-override-wide-multiply="$WIDEMUL" --with-bignum="$BIGNUM" --with-asm="$ASM" \
     --enable-ecmult-static-precomputation="$STATICPRECOMPUTATION" --with-ecmult-gen-precision="$ECMULTGENPRECISION" \
     --enable-module-ecdh="$ECDH" --enable-module-recovery="$RECOVERY" \
     --host="$HOST" $EXTRAFLAGS