util: Move util/string.h functions to util namespace

There are no changes to behavior. Changes in this commit are all additions, and
are easiest to review using "git diff -U0 --word-diff-regex=." options.

Motivation for this change is to keep util functions with really generic names
like "Split" and "Join" out of the global namespace so it is easier to see
where these functions are defined, and so they don't interfere with function
overloading, especially since the util library is a dependency of the kernel
library and intended to be used with external code.

src/util/moneystr.cpp

@@ -13,6 +13,9 @@
 #include <cstdint>
 #include <optional>
 
+using util::ContainsNoNUL;
+using util::TrimString;
+
 std::string FormatMoney(const CAmount n)
 {
     // Note: not using straight sprintf here because we do NOT want

src/util/strencodings.h

@@ -122,7 +122,7 @@ T LocaleIndependentAtoi(std::string_view str)
     static_assert(std::is_integral<T>::value);
     T result;
     // Emulate atoi(...) handling of white space and leading +/-.
-    std::string_view s = TrimStringView(str);
+    std::string_view s = util::TrimStringView(str);
     if (!s.empty() && s[0] == '+') {
         if (s.length() >= 2 && s[1] == '-') {
             return 0;

src/util/string.cpp

@@ -7,8 +7,10 @@
 #include <regex>
 #include <string>
 
+namespace util {
 void ReplaceAll(std::string& in_out, const std::string& search, const std::string& substitute)
 {
     if (search.empty()) return;
     in_out = std::regex_replace(in_out, std::regex(search), substitute);
 }
+} // namespace util

src/util/string.h

@@ -16,6 +16,7 @@
 #include <string_view> // IWYU pragma: export
 #include <vector>
 
+namespace util {
 void ReplaceAll(std::string& in_out, const std::string& search, const std::string& substitute);
 
 /** Split a string on any char found in separators, returning a vector.
@@ -162,5 +163,6 @@ template <typename T1, size_t PREFIX_LEN>
     return obj.size() >= PREFIX_LEN &&
            std::equal(std::begin(prefix), std::end(prefix), std::begin(obj));
 }
+} // namespace util
 
 #endif // BITCOIN_UTIL_STRING_H