Enable manual shrinkage of Dict

[petvana]

Based on the discussion in #44778, there seems to be no reason to disable manual shrinkage of Dict by calling sizehint! any more. The only counterintuitive side effect (I'm aware of) is that merging two sparse Dicts may lead to a single denser one because of

julia/base/abstractdict.jl

Lines 218 to 222 in 6106e6c

	function merge!(d::AbstractDict, others::AbstractDict...)
	for other in others
	if haslength(d) && haslength(other)
	sizehint!(d, length(d) + length(other))
	end

[petvana]

_tablesz seems necessary not to rehash for newsz leading the to same 2^x size.

[vtjnash]

Might be worthwhile to make this a ranged check for newsz > oldsz * 2 || newsz < oldsz \div 2, So we only resize if it is significantly different from the size it already is? Otherwise, LGTM.

[petvana]

It has to be significantly different since it goes through _tablesz now (rounded to 2^x), as I highlighted in the comment.

[vtjnash]

Ah, right, now I understand the comment better. The _tablesz means it is a power-of-2, so it is either exact, or it is different by a least a power of 2.

[aviatesk]

Bisected this commit seems to have introduced the regression within collection benchmark suite. See 8c61f40#commitcomment-73182869 for the regressed examples.