Transaction and posting tags (and posting tags inherited from accounts)
are now converted safely to Beancount-compatible transaction and posting
metadata lines.
Unsupported chars are now hex-encoded, not just converted to dashes.
This helps keep account and commodity names unique, especially with
the equity conversion account names generated by --infer-equity when
using currency symbols.
(Those could also be converted to ISO 4217 codes, in theory, but
for now we just hex encode them, which is easier to make robust.)
Also, Beancount commodity symbols are no longer enclosed in
hledger-style double quotes.
