maps: add Merge function

[louisportay]

this function allows to merge multiple generic maps in a single one

[gopherbot]

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Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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[louisportay]

Corresponding proposal: golang/go#57191

[pawnhearts]

Good

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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[gopherbot]

Message from Jorropo:

Patch Set 1:

(1 comment)

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