Add SpannableString.overlay and .overlayAll to apply spans to substrings

[shekibobo]

The overlay methods will apply a provided span to matching substrings of the receiver. In .overlay, it will apply to only the first matching occurrence, while .overlayAll will apply the span to all matching occurrences of the substring.

Because spans are not copyable, you provide a spanBuilder lambda to generate a new span to apply to each occurrence.

This supports case-insensitive matching.

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[googlebot]

CLAs look good, thanks!

[shekibobo]

Thanks! I just expanded the tests to get a little more coverage and a bit more specificity of each case. We now test the following for both methods:

• No matches
• Multiple matches
• Case sensitivity
• Start index
• Multiple different spans applied successfully

[JakeWharton]

This is great. Thank you.

My biggest concern here is that the only thing Kotlin-specific is the fact that this is an extension function instead of a static method. I'm going to try to get this into the normal support library somewhere and then we can just hang it as an extension and alias.

[shekibobo]

Okay, sounds awesome, thanks!