Null safety

• Stable null safe release.
• Add toJson and fromJson convenience methods to Serializers.

Private classes, enum `wireNumber`, memoizable `hashCode`

• Support private Built classes. Note that private classes cannot be made
  serializable.
• Support serializing enums to ints: add wireNumber to
  @BuiltValueEnumConst.
• Support memoizing hashCode, so it's computed lazily once. Write an abstract
  getter int get hashCode; then annotate it with @memoized to turn this on
  for a built_value class.
• Trim built_value_test dependencies: depend on matcher instead of test.
• Fix enum generator error messages when value and valueOf are missing.

Fix for dollars in field names

• Fix unescaped string usages while generating ValueSourceClass.
• Fix analyzer use: don't rely on toString on types.

Fix `analyzer` lower bound

• Fix analyzer lower bound: was 0.39.0, needs to be 0.39.3.

Fix regressions mixin, tweak strict analysis compliance

• Fix regression in a corner case when determining which fields to generate
  based on mixins.
• Tweak generation changes for implicit-casts: false and
  implicit-dynamic: false. Relax casts again where possible.

Comply with strict-raw-types, implicit-casts: false, implicit-dynamic: false

• Make generated code comply with analyzer option strict-raw-types.
• Allow Serialiers declaration to comply with strict-raw-types, or to
  continue to use raw types as before.
• Make generated code comply with analyzer options implicit-casts: false
  and implicit-dynamic: false.

Add `example` folders

• Add example folders with README.md pointing to examples.

Internal cleanup

Internal: cleanup for pedantic v1.9.0 lints.

Internal cleanup

• Internal: cleanup for pedantic v1.9.0 lints.

Deps cleanup

• Internal: clean up built_value_generator -> built_value dependency;
  depend on minor instead of major version so we can in future handle tight
  coupling between the two without a major version bump to built_value.