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/common/wasm_runtime_common.c

@@ -753,7 +753,7 @@ register_module_with_null_name(WASMModuleCommon *module_common, char *error_buf,
 }
 
 WASMModuleCommon *
-wasm_runtime_load(const uint8 *buf, uint32 size, char *error_buf,
+wasm_runtime_load(uint8 *buf, uint32 size, char *error_buf,
                   uint32 error_buf_size)
 {
     WASMModuleCommon *module_common = NULL;