Standardize units in the Rust SDK

[jalextowle]

There are some functions that could return base but don't. Investigate whether or not it makes sense to update these units.

[jalextowle]

It would be good to have support for asBase = false as a standard.

[wakamex]

my first thought with this is to port over the predict_trade functionality we merged in Python a few months ago:

every predict_trade call gives you the following output:

Entity	Base	Bonds	Shares
user	-100	104.95	-100
pool	99.9952	-104.955	99.9952
fee	0.0428367	0.0449786	0.0428367
governance	0.00475964	0.00499762	0.00475964

since you now know exactly what a hypothetical trade returns, this enables calling existing functions with a different unit as input. examples of this:

1. open long with bonds as input predict_trade_test.py#L111
2. open short with base as input predict_trade_test.py#L128

discussing with @dpaiton, we may want to think more deeply about how we want this functionality to look in the Rust SDK before porting that over. maybe a discussion for how we want the SDK APIs to work? @ryangoree

[wakamex]

check this out @jrhea

[wakamex]

see the comment above for what I have in mind as a return object for each trade function (already coded in python!) @jalextowle @dpaiton

[wakamex]

moving discussion to #63 (comment)