Rename `calldata` to `funcdata`

[Yaron Wittenstein]

To avoid confusion, let's rename the calldata field of the Call Transaction to funcdata.

In the same way that verifydata is the input for verify,
The funcdata will be the Call Transaction Function input.
Later, if we have the authorize extension to SVM, its input will be named authorizedata.

On the Simple Coin Iteration #2 a new buffer will be added named sigdata.
It'll contain the Signatures used for implementing any Signature Schemes.
The only difference is that sigdata won't be tied to a specific function, but it'll be used mostly as part of the verify implementations.

To not miss, it's recommended to look for the term calldata throughout the code base and update accordingly.