proto: std feature is broken

[tony-iqlusion]

What went wrong?

The std feature added in #1452 doesn't work. /cc @romac

Steps to reproduce

$ cd proto
$ cargo test --features std

This results in the following compile errors:

$ cargo test --features std                                                                                       130 ↵
   Compiling tendermint-proto v0.39.0 (/Users/tarcieri/src/tendermint-rs/proto)
error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> proto/src/google/protobuf/timestamp.rs:105:11
    |
105 | impl From<std::time::SystemTime> for Timestamp {
    |           ^^^ use of undeclared crate or module `std`

error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> proto/src/google/protobuf/timestamp.rs:106:26
    |
106 |     fn from(system_time: std::time::SystemTime) -> Timestamp {
    |                          ^^^ use of undeclared crate or module `std`

error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> proto/src/google/protobuf/timestamp.rs:107:65
    |
107 |         let (seconds, nanos) = match system_time.duration_since(std::time::UNIX_EPOCH) {
    |                                                                 ^^^ use of undeclared crate or module `std`

error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> proto/src/google/protobuf/timestamp.rs:155:29
    |
155 | impl TryFrom<Timestamp> for std::time::SystemTime {
    |                             ^^^ use of undeclared crate or module `std`

(truncated)

These errors don't occur when cargo test --all-features is used, hence why it wasn't caught by CI, ostensibly because when it's compiled with that some other crate is linking std.

It would be good in CI to explicitly test the std feature in isolation.

Definition of "done"

cargo test --features std compiles in isolation.

[romac]

Thanks to cargo-hack I realized there are other features which cannot be used in isolation, I've pushed fixes for all them in #1455.