Turns out we don't really need it directly for most "txn processing" AND
if we do it's usually related to some `Account`-ing related calcs; which
means we can instead just rely on the new `SymbologyCache` lookup to get
it when needed. So, basically just get rid of it and rely instead on the
`.fqme` to be the god-key to getting `MktPair` info (from the cache).
Further, extend the `TransactionLedger` to contain much more info on the
pertaining backend:
- `.mod` mapping to the (pkg) py mod.
- `.filepath` pointing to the actual ledger TOML file.
- `_symcache` for doing any needed asset or mkt lookup as mentioned
above.
- rename `.iter_trans()` -> `.iter_txns()` and allow passing in
a symcache or using the init provided one.
- rename `.to_trans()` similarly.
- delegate paper account txn processing to the `.clearing._paper_engine`
mod's `norm_trade()` (and expect this similarly from other backends!)
- use new `SymbologyCache.search()` to find the best but
un-fully-qualified fqme for a given `txdict` being processed when
writing a config (aka always try to expand to the most verbose `.fqme`
possible).
- add a `rewrite: bool` control to `open_trade_ledger()`.
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