Adding Parser.peek_explicit/implicit_tag()

[orthecreedence]

The idea behind this change is such that when encoding an enum (via
CHOICE) using tagging, you can determine which variant to read by
peeking at the tag value without advancing the parser.

peek_explicit_tag() and peek_implicit_tag() return a tuple like

(<raw-tag>, <decoded-tag>)

such that raw-tag is the tag as it is read directly from the input
bytes, and decoded-tag is the tag converted into the same format that
would be passed to Writer.write_explicit_element() or
Writer.write_implicit_element().

peek_implicit_tag() requires knowing the type you wish to decode
up-front in order to retrieve an accurate decoded-tag value, as
implicit tagging embeds the type within the tag.

[alex]

Can you help me understand the use case for this vs. using read_implicit_optional_element?

[orthecreedence]

Yes the case is I didn't really understand how read_optional_implicit/explicit_element worked before I went stampeding into a PR. I don't think they are terribly different!

The only real difference is the peek_*_tag methods let you match { } on the tag, but I personally don't think that's worth the cost of duplication here. Thanks for pointing me in the right direction.