`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.
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