Schema names might not be Java valid identifiers

Nobody has claimed this yet.

The `FastSerializerGenerator` calls `Schema.getName()` to create temporary variable names when iterating over some arrays and maps. If the element type is a union, then the name created by `Schema.UnionSchema.getName()` is not a valid Java identifier. The generated code causes compiler errors that look like this:
```
/tmp/generated16917366869434796488/com/linkedin/avro/fastserde/generated/serialization/AVRO_1_11/Record_GenericSerializer_1168413672.java:97: error: ']' expected
CharSequence union[null, string]0 = null;
```

This is only a problem starting with `org.apache.avro:avro:1.12.0`, which was released last month: https://github.com/apache/avro/commit/ed2dbd4d871e9ab1992c39e38d9d56f33ce55127#diff-22eb7312aa7f632f0f9b1c3f40de619f91a303ee39f43d3a62a338adf745d1efR1289 . (There are other test failures related to logical types that will need to be fixed for full Avro 1.12.0 compatibility, but this was the only issue causing compilation errors.)

Start in FastSerializerGenerator and trace how Schema.getName() is used for temporary variable names, then reproduce the failure with an element type that is a union under org.apache.avro:avro:1.12.0. The work is done when generated Java uses valid identifiers for these names and the generated serializer compiles without the reported error.

Written by the indexing model from the issue text.