RFR: JDK-8264221: Rewrite confusing stream API chain in SnippetMaps

[Tagir F.Valeev]

On Fri, 26 Mar 2021 00:28:15 GMT, Tagir F. Valeev <tvaleev at openjdk.org> wrote:

> I also simplified a couple of other places: replaced `stream().forEach()` with just `forEach()` and removed redundant `length()` argument in `substring()` call.

@lahodaj could you please review this? Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/3209