Give MBinary a common base class and expose the default length meta-attr.
Use TClass to estimate field width, not AkType.
Decimal is not fixed size 8.
For compatibility, say that all integers are 8 for now.
merge MMcM: Change how field sizes are estimated during costing to be based on TClass

Merge pull request #1105 from MMcM/sql-layer/tclass-type-sizes
https://github.com/FoundationDB/sql-layer/pull/1105