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.

Fix _finalizeBuilder

• Fix _finalizeBuilder generation so it uses the correct class name.

Defaults, process-on-build and bug fixes

• Support _initializeBuilder hook in value types. If found, it's executed
  when a Builder is created and can be used to set defaults.
• Support _finalizeBuilder hook in value types. If found, it's executed
  when build is called and can be used to apply processing to fields.
• Add defaultCompare and defaultSerialize to @BuiltValue. They
  control the defaults for compare and serialize in @BuiltValueField.
• Add facility to merge Serializers instances via Serializers.merge,
  SerializersBuilder.merge and SerializersBuilder.mergeAll.
• Bug fix: fix generated code with polymorphism and more than one level of
  non-instantiable classes.
• Bug fix: fix generated operator== when a field is called other.
• Fix num deserialization so it does not always convert to double.
• Bump version of analyzer.
• Internal: replace analyzer's DartType.displayName with custom code
  generator.

Fix codegen for custom builders with import prefixes.

• Fix codegen for custom builders when fields use types that have an import
  prefix.
• Bump version of analyzer_plugin.

"No raw types" compatibility

• Generate code compatible with 'no raw types'.

Allow providing `toString` via a mixin

• Allow providing your own toString via a mixin.
• Fix BuiltValueSerializer(serializeNulls: true) for non-primitive fields.
• Stop using old analyzer API; require analyzer 0.34.0.