Replace all remaining `.Call()` expressions with calls to `_impl` functions

[krlmlr]

Only where possible. Do not introduce new _impl functions.

For each .Call() not replaced, add a comment in the code explaining the reason, in the same line as the .Call() (at the end like .Call(...) # reason or after the opening parenthesis like .Call( # reason) .

[krlmlr]

@copilot: The assessment is extremely monotonous and also wrong: by now, all of igraph's API is available as _impl functions by very similar names (e.g.,R_ instead of Rx_) but perhaps slightly different semantics. Try harder, please.