perf: allow pre-allocate of size when new map

[VarusHsu]

There is a simple result from the bench test:

entries	withInitialedSize	withoutInitialedSize	rate
1e3	41153 ns/op	72909 ns/op	56.44%
1e5	7523395 ns/op	9731575 ns/op	77.31%
1e7	2005930041 ns/op	2555268041 ns/op	78.50%

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This change is 

[VarusHsu]

I have merged it into one commit.