ereport quality of life improvements

[hawkw]

This branch makes several improvements to the packrat ereport APIs based on experience using them. It consists of three commits that make independent changes but which depend on each other. I'd like to rebase-merge this, and the three commits can be reviewed independently in series. The changes here are:

1. packrat: return an error if an ereport was lost (183d155)

   This commit changes task-packrat-api's deliver_ereport IPC to return
   an error if the ereport did not fit in the buffer, rather than being
   infallible. In the future, this may allow some tasks to attempt
   re-delivery periodically until there's space.

2. packrat: Add helper API for serde ereports (49710b0)

   In practice, so far, most ereport payloads have been defined as structs
   implementing serde::Serialize and encoded using the minicbor-serde
   adapter crate. This requires a bit of boilerplate code, so it seems
   worthwhile to provide a method that just does that. This commit adds one
   in task-packrat-api. It's feature-flagged so we can avoid compiling
   minicbor, minicbor-serde, and serde in tasks that don't submit
   ereports using this.

3. cpu_seq: Encode ereports into static buffers (dc47a60)

   In this comment, @cbiffle reminded me that we can use
   ClaimOnceCell to use a static to create byte buffers for encoding
   ereports. This way, the buffer is never on the stack, reducing the risk
   of stack overflows when recording an ereport.

   This commit changes drv_gimlet_seq_server and drv_cosmo_seq_server
   to use that technique for their ereports.

[mkeeter]

A few nits, but looks reasonable!