[WIP] Use BigInt for I64 FFI

[gkaracha]

Work in progress. When finished, this should resolve #718 (and also fix #557).

[gkaracha]

@TerrorJack surprisingly (to me) running test primitive with this PR currently hangs:

$ stack test asterius:primitive
asterius> test (suite: primitive)

[INFO] Compiling test/primitive/primitive.hs to WebAssembly
[INFO] Converting linked IR to binaryen IR
[INFO] Running binaryen optimization
[INFO] Validating binaryen IR
[INFO] Writing WebAssembly binary to "test/primitive/primitive.wasm"
[INFO] Writing JavaScript runtime modules to "test/primitive"
[INFO] Writing JavaScript loader module to "test/primitive/primitive.wasm.mjs"
[INFO] Writing JavaScript req module to "test/primitive/primitive.req.mjs"
[INFO] Writing JavaScript entry module to "test/primitive/primitive.mjs"
[INFO] Running test/primitive/primitive.mjs

Could you test whether you can reproduce this behavior locally too?