astutils.h: reserve std::vector space in visitAstNodes() to avoid excess allocations

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This is an intermediate solution until SmallVector is able to avoid these allocation when not utilizing Boost. As it already provides a sizable improvement without the need for any external dependency I decided to go with this for now.

Testing an -O2 build with --enable=all --inconclusive on mame_regtest code (with some additional local speed hacks applied):
GCC 11 37,875,483,481 -> 33,842,501,109
Clang 13 35,459,791,724 -> 28,694,901,787