Merge #20464: refactor: Treat CDataStream bytes as uint8_t

fa29272459 Remove redundant MakeUCharSpan wrappers (MarcoFalke)
faf4aa2f47 Remove CDataStream::Init in favor of C++11 member initialization (MarcoFalke)
fada14b948 Treat CDataStream bytes as uint8_t (MarcoFalke)
fa8bdb048e refactor: Drop CDataStream constructors in favor of one taking a Span of bytes (MarcoFalke)
faa96f841f Remove unused CDataStream methods (MarcoFalke)

Pull request description:

  Using `uint8_t` for raw bytes has a style benefit:
  * The signedness is clear from reading the code, as it does not depend on the architecture

  Other clean-ups in this pull include:
  * Remove unused methods
  * Constructor is simplified with `Span`
  * Remove `Init()` member in favor of C++11 member initialization

ACKs for top commit:
  laanwj:
    code review ACK fa29272459
  theStack:
    ACK fa29272459 🍾

Tree-SHA512: 931ee28bd99843d7e894b48e90e1187ffb0278677c267044b3c0c255069d9bbd9298ab2e539b1002a30b543d240450eaec718ef4ee95a7fd4be0a295e926343f

src/test/streams_tests.cpp

@@ -149,7 +149,7 @@ BOOST_AUTO_TEST_CASE(bitstream_reader_writer)
 
 BOOST_AUTO_TEST_CASE(streams_serializedata_xor)
 {
-    std::vector<char> in;
+    std::vector<uint8_t> in;
     std::vector<char> expected_xor;
     std::vector<unsigned char> key;
     CDataStream ds(in, 0, 0);