Multi-threaded type inference

[acl-cqc]

This now based off #59 which should come first, however, I'm not sure about all of the merge:

• merge commit e6e37a9 has 22 failures, being the merge of previous 7c20289 (only 18 failures) with da57ef6 from feat: Allow writing holes which are solved by BRAT #59. I haven't identified what the 4 extra failures are but one of them is:
• examples/infer.brat now fails with "Trying to remove Hope not in set: ". We probably need to think about RemoveHope, taken from feat: Allow writing holes which are solved by BRAT #59, this may not be correct with what we're doing here in this PR?
• In Helpers.hs I put the getThunks check for hopes in vecLayers in order to share it between Braty and Kerny. This might be wrong given check' Hope raises an error for hopes in kernel code! So either (a) move the check for hopes back into getThunks or (b) allow hopes in kernels? In which case we probably want to hide that vecLayers (Helpers.hs lacks an export list)