Be less aggressive with `DroplessArena`/`TypedArena` growth.

`DroplessArena` and `TypedArena` use an aggressive growth strategy: the
first chunk is 4 KiB, the second is 8 KiB, and it keeps on doubling
indefinitely. DHAT profiles show that sometimes this results in large
chunks (e.g. 16-128 MiB) that are barely filled. Although these don't
contribute to RSS, they clog up the DHAT profiles.

This commit changes things so that the doubling stops at 2 MiB. This is
large enough that chunk allocations are still rare (you might get 100s
instead of 10s of them) but avoids lots of unused space in the worst
case. It gives a slight speed-up to cycle counts in some cases.

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