function param default values should support collection literals

[everdance]

Description

 def m(key: String,  extraParams: List[String] = []) = ...
|                                 ^
|                                 Default value must be a literal or a qualified identifier

[liufengyun]

To support it, the compiler needs to implicitly create a function that holds the default value.

The design trade-off here is how much implicit synthesis the compiler should do.

As literals and top-level functions cover most of the usage, according to the design principle of Alan Kay's rule, the best policy is to delay this.