`get` keyword behaves exactly like `method_id` (auto-calc hash),
but it's placed on the left, similar to Tact: `get T name()`.
`method_id(n)` is still valid, considering it can't be invoked by name,
since a client will compute another hash.
It's supposed it will be still used in tests and in low-level code
(not to be called externally, but to be called after replacing c3).
`get(hash)` is invalid, this keyword does not accept anything.
Note, that I have not added all builtin functions.
I filtered out strange and actually unused in practice,
like "int_at()" and similar, or "run_method0()" and similar.
(Probably, they should be dropped off even from builtins)
Also, I've modified some stdlib.fc legacy tests just to ensure
that a resulting hash doesn't change.
