util: move HasPrefix() so it can be reused

Move the function `HasPrefix()` from `netaddress.cpp` to `util/string.h`
so it can be reused by `CNetAddr` methods (and possibly others).

src/netaddress.cpp

@@ -7,8 +7,9 @@
 
 #include <hash.h>
 #include <tinyformat.h>
-#include <util/strencodings.h>
 #include <util/asmap.h>
+#include <util/strencodings.h>
+#include <util/string.h>
 
 #include <algorithm>
 #include <array>
@@ -50,13 +51,6 @@ void CNetAddr::SetIP(const CNetAddr& ipIn)
     m_addr = ipIn.m_addr;
 }
 
-template <typename T1, size_t PREFIX_LEN>
-inline bool HasPrefix(const T1& obj, const std::array<uint8_t, PREFIX_LEN>& prefix)
-{
-    return obj.size() >= PREFIX_LEN &&
-           std::equal(std::begin(prefix), std::end(prefix), std::begin(obj));
-}
-
 void CNetAddr::SetLegacyIPv6(Span<const uint8_t> ipv6)
 {
     assert(ipv6.size() == ADDR_IPV6_SIZE);