Excessive memory usage in boot and mi eval

[larshum]

The mi interpreter, and to a lesser extent also the boot interpreter, uses a significant amount of memory during the bootstrapping. Running the first bootstrapping stage with boot leads to a peak memory usage of around 1.1 GB while using the mi interpreter (as in mi eval src/main/mi-lite.mc -- 0 src/main/mi-lite.mc mi-lite) memory usage peaks at 4.4 GB. This gets even worse after #697.

I've found that one reason for this is the filename part of the info fields. I hacked together a version of the compiler where the filename is converted to a SID rather than a String (so an integer rather than a string encoding the full path of the file each AST node originates from). This led to worse runtime performance (as I said, it is a hacked-together solution), but peak memory usage drops to 0.7 GB for boot and 1.7 GB for mi eval. I think this suggests that we need to consider how we represent the filenames in our info-fields. You can find the hacked-together code at https://github.com/larshum/miking/tree/sid-hack.