Replace Reproc (#535)

Replace reproc with simpler non-dependency-based process API.

src/api/api.h

@@ -8,6 +8,8 @@
 #define API_TYPE_FONT "Font"
 #define API_TYPE_PROCESS "Process"
 
+#define API_CONSTANT_DEFINE(L, idx, key, n) (lua_pushnumber(L, n), lua_setfield(L, idx - 1, key))
+
 void api_load_libs(lua_State *L);
 
 #endif