Allow using dig.As with dig.Group

[tchung1118]

This PR adds the ability to use dig.As with dig.Group.
When dig.As is used with dig.Group, the value produced
by the constructor will be provided to the specified group
as the type specified by dig.As.

 c.Provide(newBuffer, dig.As(new(io.Reader)), dig.Group("readers"))

For example, the above code is equivalent to the following.

 c.Provide(func(...) io.Reader {
   b := newBuffer(...)
   return b
 }, dig.Group("readers"))

Fixes #341

[codecov]

Codecov Report

Merging #375 (24e9c43) into master (5ae7b1f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 24e9c43 differs from pull request most recent head 5596d66. Consider uploading reports for the commit 5596d66 to get more accurate results

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage   98.33%   98.35%   +0.01%     
==========================================
  Files          21       21              
  Lines        1441     1458      +17     
==========================================
+ Hits         1417     1434      +17     
  Misses         15       15              
  Partials        9        9              

Impacted Files	Coverage Δ
provide.go	100.00% <100.00%> (ø)
result.go	99.01% <100.00%> (+0.09%)	⬆️

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[JacobOaks]

On behalf of @r-hang

[sywhang]

looks like our GH account has some billing issues, OSPO is already working on fixing that.