Add C code generator.
Add C code generator.

It (intentionally) leaks memory like a sieve, but it can run the
PricingLexiFi.l0 benchmark.  It has some small optimisations aimed at
reducing leakage, but it's spotty.

diku-dk/futhark,mrakgr/futhark,HIPERFIT/futhark,diku-dk/futhark,ihc/futhark,ihc/futhark,mrakgr/futhark,HIPERFIT/futhark,mrakgr/futhark,HIPERFIT/futhark,CulpaBS/wbBach,diku-dk/futhark,diku-dk/futhark,ihc/futhark,diku-dk/futhark