Static update instr

[erlingrj]

This PR updates the PRET VM instructions in the C target. For portability, I argue that the right approach is to fix the data types, this is also what the JVM does. A JVM long is always 64 bit, regardless of platform.

I found a very nice way of making the instruction format flexible using union in C. Now the C implementation has the following semantics for instruction operands.

Each operand is either

1. A pointer to a 64 bit unsigned integer. No requirements on the width of the pointer itself (32bit on NRF52 and 64bit on laptops)
2. An immediate 64 bit unsigned integer. Here the width is fixed. This we need to be 64bit to encode time.

See the companion PR in reactor-c here: lf-lang/reactor-c#277

[lsk567]

Overall looks good to me. Thanks!