tendermint-proto uses std features in dependencies

[soareschen]

This is part of no_std support at informalsystems/hermes#1158. Currently the tendermint-proto crate uses the features subtle-encoding/std and chrono/clock, which depend on std.

When the relevant features are disabled in #980, we would get compile errors like follows:

error[E0599]: no method named `format_with_items` found for reference `&DateTime<Utc>` in the current scope
  --> proto/src/serializers/timestamp.rs:82:20
   |
82 |     let prefix = t.format_with_items(PREFIX.iter());
   |                    ^^^^^^^^^^^^^^^^^ method not found in `&DateTime<Utc>`

error[E0425]: cannot find function `decode_upper` in module `hex`                                                                                                            
  --> proto/src/serializers/bytes.rs:15:14                                                                                                                                   
   |                                                                                                                                                                         
15 |         hex::decode_upper(&string)                                                                                                                                      
   |              ^^^^^^^^^^^^ not found in `hex`                                                                                                                            
                                                                                                                                                                             
error[E0425]: cannot find function `decode` in module `hex`                                                                                                                  
  --> proto/src/serializers/bytes.rs:16:31                                                                                                                                   
   |                                                                                                                                                                         
16 |             .or_else(|_| hex::decode(&string))                                                                                                                          
   |                               ^^^^^^ not found in `hex`         

[soareschen]

This has now been fixed.