Fix type instability when h is a float

[maltezfaria]

On commit 41dc041, the following code errors on my machine:

@inferred extrapolate(x -> sin(x)/x, 1.0)

while this one works

@inferred extrapolate(x -> sin(x)/x, 1)

I poked around a bit, and it appears that type instability comes from the branch with x0 infinite, but I failed to really understand what was going on (I suspect it had something to do with the recursive call). This PR fixes the issue by factoring out the logic of extrapolate into an _extrapolate function that takes only positional arguments, thus avoiding the recursive call.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (41dc041) to head (616cfe2).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          101       104    +3     
=========================================
+ Hits           101       104    +3     

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