refactor: MultiAbigen rework

[prestwich]

Draft status until I update docs and changelog

Motivation

• allow distinguishing between binding generation and serialization/write failures by introducing an explicit build step
• allow lib generation
• allow building bindings in memory to prevent unnecessary disk usage on ensure_consistent

Solution

• refactor of MultiAbigen.
• MultiAbigen is now a thin vec wrapper that must be explicitly built into a MultiBindings (rather than implicit while writing)
• MultiBindings holds a map of contract name -> ContractBinding
• MultiBindings generates file contents in memory, and makes minimal disk writes
• MultiBindings no longer writes a tmp dir when running its checks
• Multibindings::ensure_consistent_crate and ::ensure_consistent_module are now available outside of #[cfg(test)] as they no longer use a tmp dir

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[mattsse]

lgtm,
think this covers all use cases

[prestwich]

Just added some additional convenience:

• impl FromIterator for MultiAbigen
• Abigen::from_file

[gakonst]

Amazing work