Implement ext_ hashes for contracts (issue #5258) (#5326)

* Implement ext_ hashes for contracts (issue #5258)

* load cryto hash .wat from raw string literal instead of file

* update .wat contents for testing crypto hashes

* remove unnecessary 'static

* fix bug in input (call_indirect required 1+ at least it seems)

* no longer use scratch buffer for crypto hash functions

* improve doc comments of ext_ hash functions

* remove unnecessary comment in .wat test file

* add return value (const 0) to contract test to hopefully enable result buffer

* fix bug in contract assertion

* implement proper output_len in contract

* implement proper test for crypto hashes

* bump spec_version 238 -> 239

* fix COMPLEXITY description

* remove final invalid instances of scratch buffer from docs