Error handling stricter, serialization

0.3.5 (2019-12-06)

• Feature: Added std:measure_time _unit_ _function_ for simple benchmarking.
• Feature: Made optional '=' in !@export and !@import statements.
• Incompatible Change: Error values are not directly assignable to data
  structure fields anymore. So that your data structures don't end up with
  errors in it, without knowing where they were actually assigned.
• Change: VVal::s_raw() returns an empty string now for VVal::Nul instead of $n.
• Bugfix: Assigning to lists and maps inside a std:to_drop marked value
  works now too.
• Bugfix: The parser didn't handle quoted symbols (:"foo") not correct and
  just returned them as string.
• Doc: Documented closures briefly, reference data types and how to do
  object oriented programming.
• Doc: Documented more reference handling.
• Feature: Added std:write\_str and added std:to_drop string representation.
• Feature: Added std:deser:csv simple but working CSV parser that copes
  also with multiline Excel-CSV exports. It deserves it's own crate probably.
• Feature: Added std:ser:csv which is a simple but working VVal table
  to CSV serializer that also handles quoting " in CSV files correctly.
• Feature: Added splice syntax for splicing vectors and maps into literals:
  $[1,2,*$[3,4]] and ${a = 1, b = 2, *${c = 3}, d = 4}.