Remove caching and simplify method args and returns

This PR removes the internal caching of instances and schemas, and simplifies the args and/or return values of several methods by removing lazy-loading. The built-in caching and lazy-loading added too much complexity for too little benefit. Callers should implement more appropriate special-purpose caching if ever needed.

This is a breaking API change, which is acceptable since Go La Tengo is still pre-1.0. The following method signatures or return semantics changed:

NewInstance() no longer de-duplicates instances, and no longer errors upon creating a duplicate instance with different auth or params.
Instance.HasSchema() now returns an error if its query fails
The following methods now take string args instead of structs:
- Instance.ShowCreateTable()
- Instance.TableSize()
- Instance.TableHasRows()
- Instance.DropSchema()
- Instance.AlterSchema()
- Instance.DropTablesInSchema()
- Instance.CloneSchema()
The following methods no longer possibly return an error:
- NewSchemaDiff()
- Schema.TablesByName()
- Schema.Table()
Schema.OverridesServerCharSet() now takes an Instance as an arg
The following methods have been replaced with identically-named exported fields:
- Schema.Tables()
- Table.CreateStatement()
The following methods have been removed entirely, as they are no longer needed now that there's no internal caching:
- Schema.PurgeTableCache()
- Schema.CachedCopy()

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ddl.go	28	29	96.55%
instance.go	265	294	90.14%
<!--	Total:	313	343	91.25%	-->

Files with Coverage Reduction	New Missed Lines	%
instance.go	3	84.86%
<!--	Total:	3		-->

Totals
Change from base Build 9:	0.7%
Covered Lines:	1369
Relevant Lines:	1587

skeema / tengo

Remove caching and simplify method args and returns #7

Pull Request Test Coverage Report for Build 13

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