Add sleep builtin

[konsolebox]

Hi Chet,

The sleep command is often used in loops and using the external sleep
is expensive.  Perhaps we can add the sleep builtin but have it
disabled by default to avoid syntax conflict with the external one.
Users who would want the builtin version can simply run:

[[ BASH_VERSINFO -ge 5 ]] && enable sleep

I have patch attached as file, and also uploaded in
https://github.com/konsolebox/bash/tree/builtin_sleep.

-- 
konsolebox

builtin_sleep.diff
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