Do not allow / in highlighter names as it is used for hierachies

/ are replaced with <slash> in the highlighter names.
Fixes #553

src/highlighter_group.cc

@@ -14,6 +14,8 @@ void HighlighterGroup::highlight(const Context& context, HighlightFlags flags,
 
 void HighlighterGroup::add_child(HighlighterAndId&& hl)
 {
+    hl.first = replace(hl.first, "/", "<slash>");
+
     if (m_highlighters.contains(hl.first))
         throw runtime_error(format("duplicate id: '{}'", hl.first));
 

src/string.cc

@@ -241,6 +241,22 @@ String indent(StringView str, StringView indent)
     return res;
 }
 
+String replace(StringView str, StringView substr, StringView replacement)
+{
+    String res;
+    for (auto it = str.begin(); it != str.end(); )
+    {
+        auto match = std::search(it, str.end(), substr.begin(), substr.end());
+        res += StringView{it, match};
+        if (match == str.end())
+            break;
+
+        res += replacement;
+        it = match + (int)substr.length();
+    }
+    return res;
+}
+
 Optional<int> str_to_int_ifp(StringView str)
 {
     unsigned int res = 0;
@@ -503,6 +519,8 @@ UnitTest test_string{[]()
     kak_assert(str_to_int(to_string(INT_MIN)) == INT_MIN);
     kak_assert(str_to_int("00") == 0);
     kak_assert(str_to_int("-0") == 0);
+
+    kak_assert(replace("tchou/tcha/tchi", "/", "!!") == "tchou!!tcha!!tchi");
 }};
 
 }

src/string.hh

@@ -280,6 +280,8 @@ String unescape(StringView str, StringView characters, char escape);
 
 String indent(StringView str, StringView indent = "    ");
 
+String replace(StringView str, StringView substr, StringView replacement);
+
 template<typename Container>
 String join(const Container& container, char joiner, bool esc_joiner = true)
 {