Remove Value_Buffer.len field
Remove Value_Buffer.len field

It is not used.

Also change `buffer` field type to avoid confusion.

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm