Add special lifting semantics for isnull() with map() and broadcast()

[nalimilan]

This is needed to allow isnull.(x) to return something useful.
Currently, the nullable_broadcast_eltype() method is needed since inference
does not work on lifted functions (we need to call it on the original function).

Fixes #180.

@davidagold @johnmyleswhite Cf. our previous discussion at #166 (comment). I think #180 makes it clear we need something like this soon.

[codecov-io]

Codecov Report

Merging #185 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #185     +/-   ##
=========================================
+ Coverage   64.56%   64.86%   +0.3%     
=========================================
  Files          13       13             
  Lines         697      703      +6     
=========================================
+ Hits          450      456      +6     
  Misses        247      247

Impacted Files	Coverage Δ
src/map.jl	92% <100%> (+0.69%)	✅
src/broadcast.jl	94.87% <100%> (+0.27%)	✅
src/lift.jl	96.96% <100%> (+0.19%)	✅

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[nalimilan]

Will merge this week-end if nobody objects.