string escaping support functions

the split function now takes an additional escape parameter and
does not split on separators that have the escaper before it.

An utility escape function that adds escape before separators
is also added.

src/string.cc

@@ -5,7 +5,7 @@
 namespace Kakoune
 {
 
-std::vector<String> split(const String& str, char separator)
+std::vector<String> split(const String& str, char separator, char escape)
 {
     auto begin = str.begin();
     auto end   = str.begin();
@@ -13,12 +13,40 @@ std::vector<String> split(const String& str, char separator)
     std::vector<String> res;
     while (end != str.end())
     {
-        while (end != str.end() and *end != separator)
-            ++end;
-        res.push_back(String(begin, end));
-        if (end == str.end())
-            break;
-        begin = ++end;
+        res.emplace_back();
+        String& element = res.back();
+        while (end != str.end())
+        {
+            auto c = *end;
+            if (c == escape and end + 1 != end and *(end+1) == separator)
+            {
+                element += separator;
+                end += 2;
+            }
+            else if (c == separator)
+            {
+                ++end;
+                break;
+            }
+            else
+            {
+                element += c;
+                ++end;
+            }
+        }
+        begin = end;
+    }
+    return res;
+}
+
+String escape(const String& str, char character, char escape)
+{
+    String res;
+    for (auto& c : str)
+    {
+        if (c == character)
+            res += escape;
+        res += c;
     }
     return res;
 }

src/string.hh

@@ -84,7 +84,8 @@ inline String operator+(Codepoint lhs, const String& rhs)
     return String(lhs) + rhs;
 }
 
-std::vector<String> split(const String& str, char separator);
+std::vector<String> split(const String& str, char separator, char escape = 0);
+String escape(const String& str, char character, char escape);
 
 inline String operator"" _str(const char* str, size_t)
 {

src/unit_tests.cc

@@ -120,11 +120,15 @@ void test_string()
 {
    kak_assert(String("youpi ") + "matin" == "youpi matin");
 
-   std::vector<String> splited = split("youpi:matin::tchou", ':');
+   std::vector<String> splited = split("youpi:matin::tchou\\:kanaky:hihi\\:", ':', '\\');
    kak_assert(splited[0] == "youpi");
    kak_assert(splited[1] == "matin");
    kak_assert(splited[2] == "");
-   kak_assert(splited[3] == "tchou");
+   kak_assert(splited[3] == "tchou:kanaky");
+   kak_assert(splited[4] == "hihi:");
+
+   String escaped = escape("youpi:matin:tchou:", ':', '\\');
+   kak_assert(escaped == "youpi\\:matin\\:tchou\\:");
 }
 
 void test_keys()