Operator assignment, Formatting and more Documentation

In this release there have been some new operators added. Most importantly
assignment operators .x += 10. Also new operators for function application &@>
have been added and for collection appending/prepending +> and <+.
Another big addition is the string formatting $F"x={:8.2}" 10.32432, which finally
makes string formatting a lot easier than by using just std:str:cat.
Also check out the reference documentation, most functions now have at least
a short description and example.

• Bugfix: Selectors for recursive $S(**/..) did not process node conditions
  right.
• Bugfix: !@import wlambda did not work as expected, it should have the same
  meaning as !@wlambda.
• Bugfix: $i(...) and $f(...) constructions were done in the wrong order
  if arguments were on the stack.
• Bugfix: If a call in RPCHandle paniced, there was absolutely no error message.
• Change: Give proper error if a numeric vector is mutated.
• Change: Numerical vectors serialize to lists of numbers now instead some
  custom Rust data structure serialization.
• Change: $code now parses blocks directly. This means $code { ... } will
  only contain the text inside the curly bracktes but not the brackets themself.
  This allows for more idiomatic std:thread:spawn $code { ... } syntax.
• Feature: Added negative key match to recursive selectors $S(**!key=childs)
  to prohibit it to recurse into certain keys.
• Feature: Added positive value condition to recursive selectors $S(**=:{x=10})
  to select only specific values to recurse into.
• Feature: Added std:bytes:replace function.
• Documentation: Color conversion functions std:v:rgb2hsv, std:v:hsv2rgb
  and others.
• Documentation: Added documentation for std:values, std:thread:spawn,
• Documentation: Completely documented the currently implemented threading support.
  fvec, ivec, is_ivec, is_fvec, is_nvec, std:str:find, std:ref_id
  and many more.
• Feature: Added debug print function special value, that includes source position and
  dynamic type name: $DEBUG ....
• Feature: Added std:time:now, std:srand and std:rand.
• Feature: Introduced variable and field assignment
  operator syntax .x += 10 and o.x += 10.
• Feature: Added apply operators &@> and <@&.
• Feature: Added collection addition operators +> and <+.
• Feature: std:values also returns the individual elements of a numerical vector.
• Feature: std:keys also returns the keys of a numerical vector.
• Feature: Implemented std:str:find.
• Feature: Implemented std:write_str.
• Feature: Implemented custom WLambda string formatting, that even supports
  writing numerical vectors, vectors or maps in a nicer formatted way.
  See also $F and std:formatter.