Unroll secp256k1_fe_(get|set)_b32 for 5x52.
Unroll secp256k1_fe_(get|set)_b32 for 5x52.

field_get_b32: min 0.647us / avg 0.666us / max 0.751us
field_set_b32: min 0.551us / avg 0.571us / max 0.624us

becomes

field_get_b32: min 0us / avg 0.0000000477us / max 0.000000238us
field_set_b32: min 0us / avg 0.0000000238us / max 0.000000238us

(Patch from https://bitcointalk.org/index.php?topic=1740973.0
 _get was reversed from the patch because this order appeared
 somewhat faster in testing.)

Signed-off-by: Gregory Maxwell <79e2475f81a6317276bf7cbb3958b20d289b78df@xiph.org>

pmienk/secp256k1,pmienk/secp256k1,apoelstra/secp256k1,apoelstra/secp256k1,digitalbitbox/secp256k1,ElementsProject/secp256k1-zkp,bitcoin-core/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,libbitcoin/secp256k1,afk11/secp256k1,libbitcoin/secp256k1,evoskuil/secp256k1,bitcoin/secp256k1,afk11/secp256k1,digitalbitbox/secp256k1,pmienk/secp256k1,afk11/secp256k1,pmienk/secp256k1,bitcoin-core/secp256k1,evoskuil/secp256k1,libbitcoin/secp256k1,digitalbitbox/secp256k1,voisine/secp256k1,digitalbitbox/secp256k1,voisine/secp256k1,evoskuil/secp256k1,libbitcoin/secp256k1,libbitcoin/secp256k1,afk11/secp256k1,apoelstra/secp256k1,apoelstra/secp256k1,voisine/secp256k1,voisine/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,bitcoin/secp256k1,pmienk/secp256k1,bitcoin/secp256k1,voisine/secp256k1,peterdettman/secp256k1,apoelstra/secp256k1,peterdettman/secp256k1,digitalbitbox/secp256k1,afk11/secp256k1,evoskuil/secp256k1