fluent-bundle 0.15.3

• This is a 'safe harbor' release prior to bringing on non-Mozilla community maintainers
• Implement From<&String> for FluentValue
• Add FluentValue.into_string to prevent String clone
• Fix FluentValue::try_number accepting numbers
• Allow optional arguments on FluentValue
• Fix behavior of FluentArgs::set
• Resolve function instead in impl ResolveValue
• Add type alias for concurrent FluentBundle
• Fix FluentBundle::format_pattern lifetimes
• Workspace: Update to Rust 2021
• Workspace: Add various missing documentation and fix typos and links
• Workspace: Cleanup meta-data using workspaces, use SPDX licenses, etc.
• Workspace: Apply rustfmt and clippy lint fixes