Add function to return slippage estimate

[bigboydiamonds]

In order to illustrate when a transaction may fail due to a priceLimit too low/high (depending on long/short), we need a function on OverlayV1Estimate to return the slippage estimate of a position attempting to be built/unwound.

[mikeyrf]

bid() and ask() functions now offer this but should change to accept and oi amount as input, instead of fraction of oi cap as we currently have.