Feature: Add chaining calls

[pault-t-canva]

Currently all calls take place in blocks of call. All data must run out for the first call to cede control to the next call in the chain. It could be useful to be able to chain calls in a sequence, defining a call repetition. Something akin to

runHttp {
  callSet {
    call("https://example.com/!1!")
      data=FileDataSuppler("in.txt")
    }
    call("https://example.org") {
      data=FileDataSupplier("in2.txt")
    }    
}

In this case the calls in a call set would take place one after each other, call per call. So the calls here would interleave between example.com and example.org.

Once all data has run out, the call chain ends.