Add ToSql and FromSql to Signed and FixedBytes #1292

Job	Run time
feature-checks	30s
fmt	12s
clippy	38s
docs	40s
miri --no-default-features	2m 56s
wasm	20s
miri	2m 57s
miri --all-features	1m 3s
test stable --no-default-features	52s
test stable	57s
test stable --all-features	36s
test beta --no-default-features	52s
test beta	52s
test beta --all-features	32s
test nightly --no-default-features	1m 24s
test nightly	1m 26s
test nightly --all-features	46s
test 1.65 --no-default-features	1m 59s
test 1.65	1m 40s
test 1.65 --all-features	1m 52s
	23m 4s

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