Add unit tests for serialized language SEXP
Implement parsing Rserve types XT_LANG_TAG and XT_LANG_NOTAG
Implement handling Rserve type XT_UNKNOWN
Skip padding in XT_ARRAY_RAW (raw vectors)
Return parsed value and end state from QapEncoding::decode
Send CMD_eval in Rserve::eval
Use uniform naming in tests for Rserve::eval and ser_eval

All tests for the Rserve class have prefix 'io-oo-rserve'. The method
being tested follows: currently, this can be '-eval' or '-sereval'.

Tests that mock the connection and use data from a saved '.qap' file
have suffix '-mock', while those that talk to a real Rserve server
use the '-live' suffix.
Change IO::evalRserve to just wrap Rserve::eval