Implement a dig.As ProvideOption

[abhinav]

This brings back #233.

Per #235 (review),
the issues we need to resolve are,

1. dig.As seems to indicate that it's a total override of the provided
   type. The way it currently works is it appends other interfaces on
   top of whatever the constructor already returns
2. semantics of dig.Provide(func New() (Foo, io.Reader, error), dig.As(new(io.Writer).
   It currently fails due to inability to case reader to writer, but
   we'd want some extra validation here. Perhaps dig.As is only
   supported for a single type, error tuple?

Closes #197

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.47%. Comparing base (4fb70ce) to head (287b330).
Report is 123 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   98.40%   98.47%   +0.06%     
==========================================
  Files          10       10              
  Lines        1130     1179      +49     
==========================================
+ Hits         1112     1161      +49     
  Misses         13       13              
  Partials        5        5              

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[glibsm]

semantics of dig.Provide(func New() (Foo, io.Reader, error), dig.As(new(io.Writer).
It currently fails due to inability to case reader to writer, but
we'd want some extra validation here. Perhaps dig.As is only
supported for a single type, error tuple?

I see the following:

1. As you said, only restrict to one type and error (has edge cases)

2. dig.As can take a vararg which gets matched to the positional arguments (minus error). Also has edge cases and is not the way the current docs are written. Docs allow for multiple casts of the same return type (is that a required usecase?)

3. ??? Maybe there is some sort of a matrix solution where you're able to cast as many params in as many ways as you want (probably requires dig.As(...) to be nestable somehow)

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[abhinav]

Superseded by #293