update calc open short & target long

[dpaiton]

Resolved Issues

partially addresses delvtech/hyperdrive-rs#40

Description

• The calculate_open_short was written at a different time from calculate_open_long; I updated it so that they follow the same process.
  edit: this caused trouble for reasons I can't explain but probably have to do with share/base conversions. I reverted it, but left some new error stuff.

• I also added some better error checking in calculate_targeted_long so that it can safely return in what was previously considered an error state.

• I added a new random test variable, the target rate itself, in the calculate_targeted_long tests.

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• Testing
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