Version 0.5.0

There have been many changes since the last release, primarily focused on performance (up to 100x in some benchmarks). These changes caused a number of API changes, the most significant of which are listed below.

• Rename the starlark_module crate to starlark_derive.
• Rename the walk methods to trace to align to standard GC literature.
• Add derive for Trace.
• Add StarlarkAttrs derivation and scheme.
• Initial start of documentation generation (still unstable).
• More complete SmallMap API.
• Three profiling modes, heap, flame and statement.
• Changes to invoke to take an Arguments structure.
• Changed to iteration APIs.
• Many semantic improvements to non-ASCII strings.
• Refinements to types and how they work.
• Mark a few additional APIs as unsafe.
• Use the gazebo Coerce trait extensively, in particular required for some of the starlark_value macros.
• Delete dict.copy and list.copy, since they aren't in the Starlark spec.
• UnpackValue no longer takes a heap argument.