fix lambdas with arguments in reverse order
remove crash in findAndUpdate when the id isn't present in the [Context]
Keep the old name when substituting a variable and it isn't found
Add custom Show instances for some types in Testing.QuickGen.Types

Makes it both easier to debug and to read. Should also be possible to
copy paste an expression from it's string representation to, for
instance, a ghci buffer.
rename getDepth => nextLambda and fix bug in popContext by not decreasing the next lambda value
update the test file