Optimize Path.UniqueKey.create

[RealyUniqueName]

Closes #9509

• Changed module_def_extra.m_file type to Path.UniqueKey.lazy_t, which addresses server/invalidate calls Path.UniqueKey.create for every cached module #9509
• Adds cache for Path.UniqueKey.create to common context, which slightly reduces unit tests compilation times
• Adds cache for Path.UniqueKey.create to eval context, which reduces unit tests run time on eval by ~10%

[Simn]

I thought that we could just have an (opaque?) type which replaces m_file : string, and then a small API for it to get the original file and the cached unique file.

This works as well, but I don't like having closure objects on our data very much.

[RealyUniqueName]

@Simn I've added UniqueKey.lazy_t and changed m_file to this type.

[Simn]

Yeah I think this is better!