Squashed commit:
[1ea1365] Improve documentation & re-add docs builder on circleci
[cba4e06] Disable parallel building of binaryen to solve OOM on circleci
[aad1174] Add Int/Word support for JSFFI; use unified JS_VAL variant for non-reference types
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[fcc48b3] Update to ghc-8.7.20180712
[701ab03] Implement jsffi wrapper functions; add Char support, add js_putchar example to jsffi test suite
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