WASM tests update

[pepyakin]

• We no longer use legalization of JS FFI. Thus we can use i64 types in interfaces directly and we no longer need to use setTempRet0. Also, LTO have been enabled for the tests, so gas consumption improved a bit.
• All libc related functions are now with prefix ext_, e.g. _malloc became _ext_malloc. This is needed to workaround some emscripten specific behavior and might change in the future.
• New functions _ext_memmove and _ext_memset added.
• mem_copy fixes. The arguments poped in a wrong order. Also make it really return a value, as LLVM really could use it sometimes.
• Small improvement of a test ergonomics: gas checks are pushed to the end of the test functions.

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[pepyakin]

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[NikVolf]

Already discussed and partially reviewed, so quick lgtm :)