Serialize mutable collections

• Add mutable collection serializers so built_value can serialize List,
  Set and Map. It's still recommended to avoid mutable collections inside
  value types as they break hashing, comparison and caching; the intended use
  case is that you have one top level mutable collection of your immutable
  value types.

Improvements to generation

• Stop generating unnecessary new keywords.
• Stop generating explicit null checks in constructors: these are not needed
  with sound null safety.
• Stop generating checks for dynamic in generics in constructors: accidental
  use of dynamic is better avoided with lints than a runtime check.

Improved build performance

• Generate code that formats faster with current "short" style. Both versions
  are fast with the coming "tall" style.
• Mark @nullable deprecated: it does nothing and should not be used.

The build is almost 2x faster than before.

Improved build performance

• Improve build performance when there are lots of built_value classes.

Fix for new `operator==` warning

• Fix for new warning about operator== param type.

StandardJsonPlugin improvement, EnumClass fix

• In StandardJsonPlugin, add support for specifying types that should be left as List.
• Allow non alphanumeric characters in EnumClass names.

Allow serializing record fields

• Allow classes with record fields to be serialized if they use a typedef for
  the record type and install a custom Serializer for it.

Add `Int32` serializer

• Add Int32 serializer.

Bump version of `analyzer`

• Bump version of analyzer.

Remove JSON from error message, fix generation

• Remove JSON from DeserializationError message to prevent data accidentally
  leaking into logs. The JSON is still available on the DeserializationError
  object.
• Fix generation for fields that are Function types that are declared
  separately, for example in a mixin defined in another source file, and use
  named or positional parameters.