Make compiled field names camel cased

Just as Java version of protobuf does, underscore delimited field names are
converted to small camel cased names.
Since I couldn't find actual conversion logic inside protobuf java code base,
rules may differ in subtle ways.
For example, it's unclear how to cope with a name collision caused by
"camelCasedField" and "camel_cased_field".
For now, the logic is as simple as it can be and those edge cases are not
addressed upon at all.
Add test cases for integer IO greater than 32-bit