types: refactor Position and related files

[austin362667]

No.1 part of #333 to support binance futures.

renamed:    pkg/bbgo/position.go -> pkg/types/position.go
renamed:    pkg/bbgo/position_test.go -> pkg/types/position_test.go

modified:   pkg/accounting/pnl/avg_cost.go
modified:   pkg/bbgo/session.go
modified:   pkg/bbgo/tradecollector.go
modified:   pkg/bbgo/tradecollector_callbacks.go
modified:   pkg/bbgo/twap_order_executor.go
modified:   pkg/strategy/bollpp/strategy.go
modified:   pkg/strategy/grid/strategy.go
modified:   pkg/strategy/support/strategy.go
modified:   pkg/strategy/xmaker/strategy.go
modified:   pkg/types/position.go
modified:   pkg/types/position_test.go