Use accurate Event type

[asazernik]

The existing Event type would not typecheck for any correct
event-handling code; dataTransfer is not present for non-DnD events, and
value is not guaranteed to be present for non-Input events.

This change is quite safe:

1. Props the user is expected to pass in are replaced with supertypes of
   what they were before (a function taking a subtype is a supertype)

2. Props the user is given are replaced with supertypes of what they
   were before, but they should only get a type error in cases where they
   are legitimately messing up (i.e. are passing an Event that doesn't have
   an HTMLInputElement as its target to the handlers in FieldProps).

[codecov]

Codecov Report

Merging #4375 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4375   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          74      74           
  Lines        1686    1691    +5     
======================================
+ Hits         1686    1691    +5

Impacted Files	Coverage Δ
src/createFieldProps.js	100% <ø> (ø)	⬆️
src/events/getValue.js	100% <100%> (ø)	⬆️

---

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[codecov]

Codecov Report

Merging #4375 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4375   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          74      74           
  Lines        1686    1691    +5     
======================================
+ Hits         1686    1691    +5

Impacted Files	Coverage Δ
src/createFieldProps.js	100% <ø> (ø)	⬆️
src/events/getValue.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 84359b3...02ec9b3. Read the comment docs.

[codecov]

Codecov Report

Merging #4375 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4375   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          74      74           
  Lines        1686    1691    +5     
======================================
+ Hits         1686    1691    +5

Impacted Files	Coverage Δ
src/createFieldProps.js	100% <ø> (ø)	⬆️
src/events/getValue.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 84359b3...325d0bf. Read the comment docs.

[codecov-io]

Codecov Report

Merging #4375 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4375   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          74      74           
  Lines        1694    1691    -3     
======================================
- Hits         1694    1691    -3

Impacted Files	Coverage Δ
src/createFieldProps.js	100% <ø> (ø)	⬆️
src/events/getValue.js	100% <100%> (ø)	⬆️
src/structure/immutable/index.js	100% <0%> (ø)	⬆️
src/structure/immutable/deepEqual.js	100% <0%> (ø)	⬆️
src/handleSubmit.js	100% <0%> (ø)	⬆️
src/createReduxForm.js	100% <0%> (ø)	⬆️
src/structure/plain/deepEqual.js	100% <0%> (ø)	⬆️
src/propTypes.js	100% <0%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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