Construct SimSprite and initialize pointer members

[jquast]

A series of memory violations, each found with the new make asan build:

• the SimSprite allocated by newPtr in newSprite() is left unconstructed
• Micropolis::callback, mapBase and mopBase are read before assignment

#12 contains the same fixes, and this PR also Closes #11. It allocates the sprite with new SimSprite(); this constructs it over the pooled allocation. It sets callback in the constructor; this initializes it where it is declared, along with mapBase and mopBase.

Detected by running sim smoke:

$ make asan
$ cd apps/micropolis
$ for i in $(seq 1 12); do echo "run #"$i: $(node_modules/.bin/tsx cli/entry.ts sim smoke --city /cities/kobe.cty --ticks 480 2>&1 | grep RuntimeError); done
run #1: RuntimeError: memory access out of bounds
run #2: RuntimeError: memory access out of bounds
run #3: RuntimeError: memory access out of bounds
run #4: RuntimeError: memory access out of bounds
run #5: RuntimeError: memory access out of bounds
run #6: RuntimeError: memory access out of bounds
run #7: RuntimeError: memory access out of bounds
run #8: RuntimeError: memory access out of bounds
run #9: RuntimeError: memory access out of bounds
run #10: RuntimeError: memory access out of bounds
run #11: RuntimeError: memory access out of bounds
run #12: RuntimeError: memory access out of bounds

Depends on #15 (the loader fix) and #16 (the make asan target) to run the commands above.