What kind of hall effect sensor IC should I use to convert a frequency measurement in a continuous voltage?
If you trigger a one-shot multivibrator from the digital output of a Hall sensor and low-pass filter the digital output (and buffer if necessary) you will get a voltage proportional to RPM. It is best if the multivibrator is operated from a well regulated supply voltage. 

This works best if you get many pulses in the time you expect the voltage to react to changes in angular velocity. 

Alternatively, it's always possible to do it digitally, and turn the resulting number into a voltage using a DAC.