Make sure to handle types no matter casing

[systay]

This query:

create table t1 (
 id BIGINT,
 public_key varchar(255)
) ENGINE=innodb;

was failing with:

panic: unimplemented type BIGINT
/usr/local/go/src/runtime/panic.go:969 (0x4360f5)
	gopanic: done = runOpenDeferFrame(gp, d)
go/src/vitess.io/vitess/go/vt/sqlparser/ast_funcs.go:295 (0x987aa3)
	go/vt/sqlparser.(*ColumnType).SQLType: panic("unimplemented type " + ct.Type)
go/src/vitess.io/vitess/go/vt/vtexplain/vtexplain_vttablet.go:407 (0xd0dc82)
	go/vt/vtexplain.initTabletEnvironment: Type: col.Type.SQLType(),
go/src/vitess.io/vitess/go/vt/vtexplain/vtexplain.go:158 (0xd06662)
	go/vt/vtexplain.Init: err = initTabletEnvironment(parsedDDLs, opts)
go/src/vitess.io/vitess/go/cmd/vtexplain/vtexplain.go:167 (0xd1327f)
	parseAndRun: err = vtexplain.Init(vschema, schema, opts)
go/src/vitess.io/vitess/go/cmd/vtexplain/vtexplain.go:132 (0xd12db1)
	main: err := parseAndRun()