Add Fx.Replace

[sywhang]

This adds fx.Replace, which replaces a value already provided in the graph with another value. This is similar to how fx.Supply provides an instantiated value to the graph.

For example the following code that uses fx.Decorate to modify the *bytes.Buffer type...

fx.New(
  fx.Provide(func() *bytes.Buffer { ... }),
  fx.Decorate(func() *bytes.Buffer {
    ...
  }),
  fx.Invoke(func(b *bytes.Buffer) {
    // I get a modified buffer!
  }),
)

is the same as this version that uses fx.Replace:

b := func() *bytes.Buffer { ... }

fx.New(
  fx.Provide(func() *bytes.Buffer { ... }),
  fx.Replace(b),
  fx.Invoke(func(b *bytes.Buffer) {
    // I get a modified buffer!
  }),
)

Implementation-wise, it is very similar to how fx.Supply creates a function that produces the given value using reflection and dig.Provides it - fx.Replace creates a function that produces the given value with no argument using reflection, and dig.Decorate with that function.

[codecov]

Codecov Report

Merging #837 (923c0e8) into master (9173ccd) will decrease coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
- Coverage   98.56%   98.42%   -0.15%     
==========================================
  Files          29       30       +1     
  Lines        1043     1076      +33     
==========================================
+ Hits         1028     1059      +31     
- Misses         10       11       +1     
- Partials        5        6       +1     

Impacted Files	Coverage Δ
replace.go	100.00% <100.00%> (ø)
app.go	94.95% <0.00%> (-0.92%)	⬇️

---

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[abhinav]

Minor issue in the test case.

---

Separately from this PR, we might need to tweak the value group handling a bit more.
Question: if I do this, what's the value in group:"t" at the top-level/what should it be?

foo := fx.Module("foo",
  fx.Supply(
    fx.Annotate([]string{"a", "b", "c"}, `group:"t,flatten"`),
  ),
)

fx.New(
  fx.Module("wrapfoo", 
    foo,
    fx.Replace(
      fx.Annotate([]string{"d", "e", "f"}, `group:"t"`),
    ),
  ),
)

[sywhang]

@abg thanks for catching the issue with the test. Fixed that.

Question: if I do this, what's the value in group:"t" at the top-level/what should it be?

I believe that should be []string{"d", "e", "f"}? Unless that wasn't what you were thinking. Decoration at top-level should override anything provided by child modules.