Option to check syntax only

[vporton]

DFX should support quick syntax/semantics checking of source code. Checking syntax by running dfx build is far too slow (apparently, because the compiler does optimizations).

moc already has --check option, it remains to integrate it into dfx like

dfx check [canister]

It is questionable, whether all dependent canisters should also be type-checked.

[sesi200]

We collect feature requests over here now and use upvotes to prioritize our work: https://dx.internetcomputer.org/topic/196