Rewrite the dump library to avoid intermediate heap allocations

[paulhuggett]

The YAML output code used by pstore-dump was originally written for another project. That project in general dealt with very small files of the order of a few kilobytes at most. For this reason, the code prioritised the appearance of its output over output performance.

Reusing the code in pstore-dump was expedient: it got the tool up-and-running very quickly.

However, pstore files can hold many megabytes of data meaning that the performance of the tool can be important. Currently, any command that produces a lot of output can take a long time.

For this reason, I proposed that we eliminate the intermediate objects and dump the output directly.