doctests are very slow to run

[simonmichael]

hledger-lib's doctests are taking half a minute, making stack test, make test etc. much too slow for comfort:

$ time stack test hledger-lib:test:doctests
hledger-lib-1.1.98: test (suite: doctests)

Examples: 106  Tried: 106  Errors: 0  Failures: 0


real    0m32.192s
user    0m23.860s
sys     0m4.416s

[simonmichael]

https://github.com/sol/doctest/issues/141