gen_mod_rs codegen option

[stepancheg]

When this option is specified, codegens generate mod.rs which
pub use all the generated files.

For the test included in this PR, generated $OUT_DIR/include_generated/mod.rs contains:

// @generated

pub mod v2;
pub mod v3;

This way:

• users can include generated files
• users don't have to enumerate generated files
• this opens a door to generating a mod hierarchy (foo::bar for foo/bar.proto)

If you want to review this diff (and please do review!) start with looking at protobuf-test/src/include_generated/mod.rs in this PR.

gen_mod_rs option can be specified

• in Customize struct when codegen is invoked programmatically
• or with command-line option protoc --rust_opt=gen_mod_rs=true --rust_out=... when invoking protoc command

When landed this option will be backported to the stable branch (v2), and likely will be on by default in the next stable version (v3).

[clintfred]

Will this also apply to protubuf-codegen-pure?

[stepancheg]

Yes, it will be available for all three ways to generate protobuf files: protoc-gen-rust plugin, protoc-rust and protobuf-codegen-pure.

[stepancheg]

a9f694a is in v2.18 branch. To be released soon.

[stepancheg]

2.18.0 released.