Add bindings to represent a function being called with nothing

[johnsonw]

• Update matchers with __.LastCalledWith()
• Update bindings with lastCalledWith:unit -> unit
• Add a test to ensure that we can verify a mock is invoked with nothing passed to it.

[codecov[bot]]

Codecov Report

Merging #8 into master will increase coverage by 0.15%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   91.07%   91.22%   +0.15%     
==========================================
  Files           1        1              
  Lines          56       57       +1     
  Branches        7        7              
==========================================
+ Hits           51       52       +1     
  Misses          5        5

Impacted Files	Coverage Δ
fable/Matchers.fs	91.22% <100%> (+0.15%)	:arrow_up:

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[jgrund]

Thanks!