Fix wasm_runtime_load argument type invalid issue (#1059)

Remove the `const` flag for the first argument `buf` of wasm_runtime_load as
it might be modified by runtime for footprint and performance purpose, and
update the related functions and document.

core/iwasm/interpreter/wasm_mini_loader.c

@@ -2336,7 +2336,7 @@ load(const uint8 *buf, uint32 size, WASMModule *module, char *error_buf,
 }
 
 WASMModule *
-wasm_loader_load(const uint8 *buf, uint32 size, char *error_buf,
+wasm_loader_load(uint8 *buf, uint32 size, char *error_buf,
                  uint32 error_buf_size)
 {
     WASMModule *module = create_module(error_buf, error_buf_size);