Some API improvements (still needs redesign)

Significant API changes, but the API still needs a redesign to fix soundness issues.

To be clear, this does not fix the outstanding soundness issues.

See commit notes for full list of changes.

Major Features

• Add GcError wrapper
• Add GcString as garbage collected string
• Add an "epsilon" collector that never collects memory
• Implement GcDeserialize using serde
• Optionally emulate core::alloc API
• Add GcIndexMap collection

Public API Changes

• Parameterize GcSafe by 'gc lifetime and CollectorId
• Make GcErase trait an alias GcRebrand

Internal Improvements & Fixes

• Use const generics for arrays
• Fix trait_object_trace! macro
• Make GcHandle use generic associated types
• Handle cycles in Trace::NEEDS_TRACE
• Remove #[feature(...)]s that have been stabilized