Remove the use of trait objects as errors from `from_str` and
`try_from_into`; they seem to have caused a lot of confusion in
practice. (Also, it's considered best practice to use custom error
types instead of boxed errors in library code.) Instead, use custom
error enums, and update hints accordingly. Hints also provide
some guidance about converting errors, which could be covered
more completely in a future advanced errors section.
Also move from_str to directly after the similar exercise `from_into`,
for the sake of familiarity when solving.
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