Merge 'Fix string funcs' from Nikita Sivukhin

Add fuzz test for string functions and fix 2 bugs in implementation of
`LTRIM/RTRIM/TRIM` and `QUOTE`:
- `QUOTE` needs to escape internal quote(`'`) symbols
- `LTRIM`/`RTRIM`/`TRIM` needs to check if they have additional argument

Closes #958

testing/scalar-functions.test

@@ -631,6 +631,10 @@ do_execsql_test quote-string {
   SELECT quote('limbo')
 } {'limbo'}
 
+do_execsql_test quote-escape {
+  SELECT quote('''quote''')
+} {'''quote'''}
+
 do_execsql_test quote-null {
   SELECT quote(null)
 } {NULL}