Bug 1283 asset code rendering

[graydon]

Fixes #1283 by reusing the rendered asset code prefix of metadata.name for metadata.symbol

Unfortunately creates a new host object during SAC init, which requires re-observing basically all the SAC tests.

[graydon]

(Note the other option -- trimming asset_info.asset_code and re-padding it when converting to XDR -- would probably cause a little less churn in the testsuite due to not having to make a new object; but it would still perturb a bit due to the hash of storage and the host object repertoire changing. Not sure if it matters, but I'm willing to try that variant too?)