Generalize size function to operate on any collection-typed input

[jdidion]

The size function is very useful for determining the sizes of inputs, such that the size requested in the disks runtime attribute can be determined dynamically. However, the usefulness of size is limited by the fact that it can only operate on File and Array[File]. For more complex inputs it is common to bundle multiple files together into a collection, e.g.

struct Fastqs {
  File read1
  File? read2
  Boolean? interleaved
}

task foo {
  input {
    Array[Fastqs] sample_fastqs
  }
  ...
}

I propose that we change the signature of size to:

Float size(X?, [String])

where X is either File or a compound type (Array, Map, Pair, Object, or struct) that contains File?s. The output is the sum of all sizes of all files contained within the argument value to any level of nesting.

Draft implementation: https://github.com/openwdl/wdl/tree/531-generalize-size