In order to support existing `pps.toml` files in the wild which don't
have the `asset_type, price_tick_size, lot_tick_size` fields, we need to
only optionally read them and instead expect that backends will write
the fields going forward (coming in follow patches).
Further this makes some small asset-size (vlm accounting) quantization
related adjustments:
- rename `Symbol.decimal_quant()` -> `.quantize_size()` since that is
explicitly what this method is doing.
- and expect an input `size: float` which we cast to decimal instead of
doing it inside the `.calc_size()` caller code.
- drop `Symbol.iterfqsns()` which wasn't being used anywhere at all..
Additionally, this drafts out a new replacement market-trading-pair data
type to eventually replace `.data._source.Symbol` -> `MktPair` which we
aren't using yet, but serves as the documentation-driven motivator ;)
and, it relates to https://github.com/pikers/piker/issues/467.
3a4794e9d1