Add default out_dir support

[jokemanfire]

If not set out_dir,it will use env 'OUT_DIR' to generate proto rs file.You can use 'include!' marco to include env 'OUT_DIR' proto rs .
And you needn't to create a dir in your source dir.

[Tim-Zhang]

@jokemanfire The feature is useful, and I think we'd better not update the example because I occasionally need to check the generated code and out_dir is not easy to find. By the way, please take a look at the failed CI, thanks

[jokemanfire]

@jokemanfire The feature is useful, and I think we'd better not update the example because I occasionally need to check the generated code and out_dir is not easy to find. By the way, please take a look at the failed CI, thanks

Ok ,I've changed and add usage in doc.

[Tim-Zhang]

@jokemanfire I left a new comment, PTAL, thanks

[jokemanfire]

@jokemanfire I left a new comment, PTAL, thanks

error: package `home v0.5.11` cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.77.0
Either upgrade to rustc 1.81 or newer, or use
cargo update home@0.5.11 --precise ver
where `ver` is the latest version of `home` supporting rustc 1.77.0
make: *** [Makefile:9: debug] Error 101

CI now have some problem need update rustc version ???
There 's no cargo lock so it may use the latest home package home v0.5.11 ,but it only support up v1.81 version rustc

[Tim-Zhang]

@jokemanfire After the pr #267 was merged, the rustc verstion problem has disappeared. Please rebase and remove the fix commit. thanks

[jokemanfire]

@jokemanfire After the pr #267 was merged, the rustc verstion problem has disappeared. Please rebase and remove the fix commit. thanks

Ok I've update

[Tim-Zhang]

@jokemanfire I left comments again. PTAL, thanks.