binpac fixes
----------------

numbers of issues below correspond to the patch numbers

(1) correct calculation of minimal header size in pac_expr.cc
- problem: EXPR_CALLARGS and EXPR_CASE not considered for the calculation
  of minimal header size
- solution: added two cases in switch stmt of Expr::MinimalHeaderSize
  for EXPR_CALLARGS and EXPR_CASE


(2) ensure parsing of fields first referenced in a case expression or
    let field with an &if attribute
- problem: in cases where the if expression evaluates to false or the
  proper case does not occur, fields get not parsed at all
- solution: force evaluation of all IDs referenced in a let field with
  if attribute or a case expression before the body of the corresponding
  switch stmt or the if stmt
- added public method Expr::ForceIDEval, properly called before
  generating the code of a field with if attribute or the case expression


(3) properly assert the use of fields with an if attribute
- problem: the use of fields with an if attribute was not asserted in all
  cases and asserted in the wrong way in some others due to the
  corresponding BINPAC_ASSERT only called upon parsing the field
- solution: perform BINPAC_ASSERT upon calling the fields accessor
  function
- moved BINPAC_ASSERT statement from LetField::GenEval to
  Type::GenPubDecls


(4) incremental input with records with a non-negative StaticSize
- problem: incremental input with records with a StaticSize >= 0
  cannot be performed due to necessary length attribute, leading to
  an invalid call of GenBoundaryCheck in RecordType::DoGenParseCode
- solution: added a check for incremental input in
  RecordType::DoGenParseCode before calling GenBoundaryCheck


(5) empty type with incremental input
- problem: with an empty type and incremental input, although the
  Parse function is created, it is never called, leading to problems,
  if additional actions are to be performed when encountering that
  empty type
- solution: generate call to Parse of empty type in Type::GenParseBuffer


(6) parsing loop in flow ParseBuffer (while(true))
- problem: while(true) leads to problems after parsing of a type is
  complete; at this time, it is unexpected that parsing continues, even
  if no data is available in the flow buffer
- solution: check if data is available before starting a new parsing
  cycle
- added a method data_available to FlowBuffer
- changed while(true) in FlowDecl::GenCodeFlowUnit to
  while(flow_buffer_->data_available())


(7) initialization of flow buffer in CaseType with bufferable fields
    in cases
- problem: initialization of buffer occurs in every Parse call,
  regardless if it was initialized before or not; initialization
  is correct only on first such occurrence
- solution: check to buffer_state is to be created always when
  buffering_state_id is in environment in Type::GenBufferConfig
- changed condition from buffering_state_var_field_ to
  env->GetDataType(buffering_state_id)


(8) allowing init and cleanup code to be redefined, as well as addition
    of code to FlowEOF calls in analyzer and flow
- problem 1: when refining an analyzer or flow definition, additional
  init and cleanup code was not allowed, if these were already defined
  before; this leads to problems when adding new members, as these
  cannot be initialized and destroyed properly
- solution: allow init and cleanup code to be specified more than once
- changed deifnitions and usage of constructor_helper and
  destructor_helper to allow for lists of constructor and destructor
  helpers (similar to member declarations) in pac_analyzer.h and
  pac_analyzer.cc
- problem 2: in some cases, it is desirable to execute code when
  encountering the end of the input stream, which is not possible in
  binpac
- solution: added a %eof binpac primitive similar to %init, which adds
  code to the FlowEOF function of an analyzer or a flow
