Make `@unroll` a preview feature

[odersky]

Compiler version

3.8

Expectation

SIP 61 @unroll has been accepted for shipping but is currently an experimental feature. We should make it a preview feature for 3.10.

TODOs:

In 3.10:

• Change the @experimental annotation in unroll.scala to @preview.
• Change the tests use options -preview instead of options -experimental.

In 3.11

• Drop the preview annotation on @unroll and the options -preview using clause in the tests.