refactor: re-enable UBSan implicit-sign-change in serialize.h

Made every signed/unsigned conversion in the serialization helpers explicit so the UBSan `implicit-sign-change` check passes and the `serialize.h` suppression can be dropped.

For consistency, a few other simple changes were also applied to the serialization helpers:
* remove redundant `inline` on function templates;
* unify formatting to make the differences between similar methods obvious.

test/sanitizer_suppressions/ubsan

@@ -65,7 +65,6 @@ implicit-integer-sign-change:TxConfirmStats::removeTx
 implicit-integer-sign-change:prevector.h
 implicit-integer-sign-change:verify_flags
 implicit-integer-sign-change:EvalScript
-implicit-integer-sign-change:serialize.h
 implicit-signed-integer-truncation:crypto/
 implicit-unsigned-integer-truncation:crypto/
 shift-base:arith_uint256.cpp