Rollup merge of #25703 - eddyb:lazier-loads, r=pnkfelix

Reduces pre-trans heap baseline (tested on `fn main() {}`) from 1.2MB to 696kB.
Most of the remaining allocations are used by imported macros.
While it might be possible to also load macros lazily, doing so is non-trivial and this PR doesn't tackle that.