Accept --export to pick which func to run

[jbourassa]

With --export (or -e), users can specify which function to invoke. This is similar to Wasmtime's CLI --invoke.

Some decisions:

• The flag is either --export or -e: most other flags also had long and short variants, so I followed the pattern.
• The export name is required in engine run and defaults at the CLI level: mostly because it was the easiest.
• Let Wasmtime's error message bubble: I think it's explicit enough.

$ cargo run -- -f benchmark/build/exports.wasm <(echo "{}")
Error: failed to find function export `_start`

#gsd:34464

[dphm]

Not sure how to try it out but it makes sense to me!

[jbourassa]

Not sure how to try it out

Checkout the branch and run :

cargo run -- -f benchmark/build/exports.wasm <(echo "{}") -e export1

where:

• -e or --export specify the export
• <(echo "{}") is a fancy way of specifying an empty JSON file