Fix no-std compilation

[evanlinjin]

Description

serde_json requires either std or alloc feature enabled. We just enable alloc as we don't need the features that std provides.

Notes to the reviewers

alloc is just a subset of std. json_serde has from_reader() & to_writer() methods if std is enabled. We aren't using those methods so just having alloc is fine.

I'm also not sure why an explicit alloc cargo feature flag is needed in serde_json. They can just have an internal prelude based on whether the std feature is enabled.

Checklists

All Submissions:

• I followed the contribution guidelines

[luisschwab]

ACK 546b07e

[oleonardolima]

ACK 546b07e