Use WebAssembly block locals for temporary variables
Some instructions (those that use
new_local in the interpreter generator) rely on local variables. On WebAssembly these are currently globals (which is faster than function locals because it avoids register spilling; see §5 of the IFL'19 paper). When WebAssembly multi-value is sufficiently supported, we can use blocks with inputs to have locals just for those instructions that need them.
Multi-value is also required for #90 (comment 95967); see there for support overview.