Improve debugger handling of large objects.

[baptr]

It's confusing to see <null> objects in the debugger inspector when
they should be assigned, and hard to track down why it might be.

It turns out the remote debugger's DebuggerMarshalls serialize functions
default to transferring at most 1MiB of data. Before this, it would
silently drop anything that would be too large.

• Stop recursively encoding objects when checking their size, since
  they're not actually sent that way.
• Change PROPERTY_HINT_OBJECT_TOO_BIG to a more generic
  PROPERTY_HINT_ENCODING_ERROR and use hint_string to specify the issue.
• Surface the new ENCODING_ERROR hint_string in the debugger.
• Plumb ENCODING_ERROR hints (but no others) through the remote
  debugger.

[YuriSizov]

• Change PROPERTY_HINT_OBJECT_TOO_BIG to a more generic
  PROPERTY_HINT_ENCODING_ERROR and use hint_string to specify the issue.

Just a heads up, that this is a breaking change and makes this PR unlikely to be merged.