Add specialized instructions for loading some locals.

Not having to decode a separate arg byte improves some benchmarks:

binary_trees - wren           ..........  3094  0.32s  101.95% relative to baseline
delta_blue - wren             ..........  6555  0.15s  101.05% relative to baseline
fib - wren                    ..........  2326  0.43s  107.32% relative to baseline
for - wren                    ..........  6183  0.16s  102.81% relative to baseline
method_call - wren            ..........  4746  0.21s  105.97% relative to baseline