Move unit package out of otel module

[MrAlias]

The unit package is used by the metrics module and is not stable. It needs to be relocated in a different module than otel so that module can be released as 1.0.

• Determine best place to move the unit package.
• Move the unit package.

[MrAlias]

It looks like the unit package can be moved to the metric module:

$ grep -Rn 'go.opentelemetry.io/otel/unit' | grep '\.go:'
unit/doc.go:20:package unit // import "go.opentelemetry.io/otel/unit"
unit/unit.go:15:package unit // import "go.opentelemetry.io/otel/unit"
metric/config.go:18:	"go.opentelemetry.io/otel/unit"
metric/metric.go:22:	"go.opentelemetry.io/otel/unit"
metric/metric_test.go:26:	"go.opentelemetry.io/otel/unit"
bridge/opencensus/exporter.go:34:	"go.opentelemetry.io/otel/unit"
bridge/opencensus/exporter_test.go:36:	"go.opentelemetry.io/otel/unit"