warn when using setState in constructor

[mzealey]

Obviously it could have been used by functions called from the constructor, but using setState in constructor is always an issue in react and causes a warning in debug mode.

[ljharb]

Seems reasonable.

[jenil94]

@ljharb, I would like to give a shot at it.

[ljharb]

Sounds great; anything with a help wanted label, and no existing linked PR, is up for grabs.