Add toIntOption (and Long, Boolean, et al)

[rjolly]

fixes #554

[SethTisue]

looked at this with Raphael just now and some work items remain:

• move the code to the scala_2.11_2.12 folder (hopefully a separate 2.11 version won't be needed)
• refactor to remove the duplication of assertThrow*

[SethTisue]

perhaps @martijnhoekstra, the original author, would look to take a look, once this is in final form

[SethTisue]

Looks like JS needs to expect a different exception:

[error] Test scala.collection.compat.StringParsersTest.nullDouble failed: Wrong exception: expected java.lang.NullPointerException but was scala.scalajs.js.JavaScriptException, took 0.002524921 sec [58](https://github.com/scala/scala-collection-compat/actions/runs/3417699470/jobs/5689134821#step:5:59) 

[SethTisue]

as for Native,

[error] Test scala.collection.compat.StringParsersTest.floatSpecificTest failed: str.toFloat <> str.toFloatOption for NaNd, took 0.009 sec [69](https://github.com/scala/scala-collection-compat/actions/runs/3417699470/jobs/5689134943#step:5:70) [error] Test scala.collection.compat.StringParsersTest.doubleSpecificTest failed: str.toDouble <> str.toDoubleOption for NaNd, took 0.009 sec 

[rjolly]

Seems like both JS and Native think that "NaNd" should parse, as opposed to JVM.

[SethTisue]

I added a commit that brings the readme up to date.

@scala/collections folks want to take a look before merge...?

[SethTisue]

thank you @rjolly!