no_std support still depends on std

[simpkins]

esp-idf-part still appears to depend on the std crate even when used without the std feature.

The serde_plain dependency pulls in a dependency on std, as it depends on serde with the default features.
This could be made an optional dependency, but there are still other std dependency requirements: serde appears to only support untagged enums when the std feature is enabled, and this is used for the SubType enum. It probably would be possible to put all derive(Serialize, Deserialize) attributes behind a cfg_attr check.

All that said, given that PartitionTable::try_from_bytes() is only available with the std feature I don't think that no_std support is going to be useful for what I was hoping to do anyway. The dependency on deku seems to be the main hurdle to making try_from_bytes() work with no-std and no-alloc. For my use case I know my buffer will be aligned, so it's probably easiest for me to just declare my own #[repr(C)] PartitionTableEntry struct and just use bytemuck::from_bytes() to read the table entries through that structure.