Improve debug info on parsing errors.

The name used for a container is now the name of the struct field or the PType [union/alias].
On this occasion, standardize the way parse/extract functions for containers list their args :
 - specific args
 - name
 - parse_fun
 - input
Micro-update of prepare-release.sh to produce only bytecode for tutorials.
Typofix: conditional_container.
New helper in core: string_input_of_stdin.
Initial commit on OpenPGP.
Add iteration_counter dump fun, mpint dump fun and diversification of pubkey_* enum.
Misc improvement on openpgp

 - fix a typo on iteration_counter
 - delete some commented-out obsolete code
 - add several String representations of enum entries
 - packet_len type is now a "uint32 option": this allows to represent "undetermined packet len" as None, while defined packet len are "Some int"
 - implement dump_packet_len value_of_packet_len dump_subpacket_len
 - make some spacing aligment to ease reading
 - reactivat the parse_checkpoint that validates there is at least one SigCreationTime subpacket
 - replace a magic("\x00.......\x00") by a list(X) of magic("\x00") to improve readability
Add pretty dates.

Now parse list of packets per default
Add pretty-printer for packet_version.

 - replace overly complicated packet_type value_of_* function body
 - add support for partial body len w/ a container

/!\ Support is at best experimental as it is hogging memory; on top of that, support is lossful as it drops intermediate partial len ; dump is not supported
Add support for Radix64.
Improve openpgp code.

 - add some error handling for the String.index_from and Str.search_forward function calls to prevent Not_found to bubble up with further informations when a non-armored input file is provided
 - output is now json
 - new type "conditional_string" that takes a predicate as parameter and continue to munch input until predicate values true ; allows to munch input much like drop_until but stores parsed data as a string
 - we have now a magic("-----") instead of list(5) of magic("-") just to improve readability of the output
Misc improvements on OpenPGP.

 - add named containers
 - add relaxed parsing of armor format
 - rename some field names
Bugfix on user attributes.

 - v2 packets are now handled (it is in fact the same format as v3 packets)
 - return to a binary version
 - there is still a need for a switch or a fallback
Simplify some enums using multi-patterns and ranges.
Update Changelog.