Correctly build operations via `Contract.call()`

[Shaptic]

The type of InvokeContractArgs.functionName is supposed to be a string now, not an ScSymbol like before.

This should have been caught by testing but they were being skipped with xit by mistake :facepalm:

[github-actions[bot]]

Size Change: -89 B (0%)

Total Size: 3.15 MB

Filename	Size	Change
dist/stellar-base.js	2.31 MB	-62 B (0%)
dist/stellar-base.min.js	840 kB	-27 B (0%)

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