Make mg_url_encode public

Change the API to heap-allocate the string and get rid of a bunch of fat stack buffers.

PUBLISHED_FROM=c793dc64a9a044a9769d92ea16859f5d704a2181

docs/c-api/util.h/intro.md

@@ -22,6 +22,7 @@ items:
   - { name: mg_sock_to_str.md }
   - { name: mg_start_thread.md }
   - { name: mg_stat.md }
+  - { name: mg_url_encode.md }
 ---
 
 

docs/c-api/util.h/mg_url_encode.md

@@ -0,0 +1,13 @@
+---
+title: "mg_url_encode()"
+decl_name: "mg_url_encode"
+symbol_kind: "func"
+signature: |
+  struct mg_str mg_url_encode(const struct mg_str src);
+---
+
+URL-escape the specified string.
+All non-printable characters are escaped, plus `._-$,;~()/`.
+Input need not be NUL-terminated, but the returned string is.
+Returned string is heap-allocated and must be free()'d. 
+