Added tag 0.6.0 for changeset 5d7db7c1774b
pretty: provide a `softbreak` parameter to help break long lines

If a leaf subexpression to be computed gets over the soft break,
we allow it to be broken down (or unflattened).
provide a pretty printer with a decent layout

While doing this we dissociated Symbol from Keyword.
It seems to bring more harm than value.
parallel-eval: fix exception propagation
Added tag 0.5.0 for changeset 4593af10de43
attempt at a thread-parallel evaluator
give a lispy repr to symbols and keywords
provide a `nil` mapping to `None`
Added tag 0.4.0 for changeset 87e9d4800aff
test: show #t and #f parse/serialize roundtrip
reader: detect and signal quoting issue
test: exhibit a blocking behaviour of the parser (nasty)

In fact we are looping forever in the `next_token` method.
test: show a syntactically wrong expression yielding a good value
test: assert some syntax errors
retire the GLOBALENV

Looks like a magnet for hidden bugs.