Add option KeepByteCodeSymbols to keep the symbol table when stripping bytecode

Option -s of bcstrip was added in https://gitlab.science.ru.nl/clean-and-itasks/abc-interpreter/commit/49085f9cba5c283a8680e8b4065b6ae4e4e410ec.

So there are basically three options:

1. Strip sections and symbol table (useful when you just want to interpret)
2. Strip sections but keep symbols (useful when you want to serialize/deserialize)
3. Don't strip (useful if you suspect a bug in the stripper)

Before you would need option 3 for using the serialization library, but now the most sensible default is option 2.