cmStringAlgorithms: cmIsSpace, cmTrimWhitespace, cmEscapeQuotes, cmTokenize

This adds the following functions to `cmStringAlgorithms`:

- `cmIsSpace`
- `cmTrimWhitespace` (moved from `cmSystemTools::TrimWhitespace`)
- `cmEscapeQuotes` (moved from `cmSystemTools::EscapeQuotes`)
- `cmTokenize` (moved from `cmSystemTools::tokenize` and adapted to
  accept `cm::string_view`)

Source/cmStringAlgorithms.cxx

@@ -5,6 +5,59 @@
 #include <algorithm>
 #include <cstdio>
 
+std::string cmTrimWhitespace(cm::string_view str)
+{
+  auto start = str.begin();
+  while (start != str.end() && cmIsSpace(*start)) {
+    ++start;
+  }
+  if (start == str.end()) {
+    return std::string();
+  }
+  auto stop = str.end() - 1;
+  while (cmIsSpace(*stop)) {
+    --stop;
+  }
+  return std::string(start, stop + 1);
+}
+
+std::string cmEscapeQuotes(cm::string_view str)
+{
+  std::string result;
+  result.reserve(str.size());
+  for (const char ch : str) {
+    if (ch == '"') {
+      result += '\\';
+    }
+    result += ch;
+  }
+  return result;
+}
+
+std::vector<std::string> cmTokenize(cm::string_view str, cm::string_view sep)
+{
+  std::vector<std::string> tokens;
+  cm::string_view::size_type tokend = 0;
+
+  do {
+    cm::string_view::size_type tokstart = str.find_first_not_of(sep, tokend);
+    if (tokstart == cm::string_view::npos) {
+      break; // no more tokens
+    }
+    tokend = str.find_first_of(sep, tokstart);
+    if (tokend == cm::string_view::npos) {
+      tokens.emplace_back(str.substr(tokstart));
+    } else {
+      tokens.emplace_back(str.substr(tokstart, tokend - tokstart));
+    }
+  } while (tokend != cm::string_view::npos);
+
+  if (tokens.empty()) {
+    tokens.emplace_back();
+  }
+  return tokens;
+}
+
 namespace {
 template <std::size_t N, typename T>
 inline void MakeDigits(cm::string_view& view, char (&digits)[N],