New trait for collections that change sizes (issue #22)

[Tokazama]

Even if you have a mutable structure it might not be able to change sizes. With this PR if is_dynamic returns true then the following operations should work for a given collection:

• push!
• pushfirst!
• pop!
• popfirst!
• insert!
• deleteat!

I also updated the README with these methods and can bump the version number if this all sounds good.

[ChrisRackauckas]

is_dynamic_sized?

[chriselrod]

I'm a little concerned about ambiguity of dynamic referring to changing size vs size-unknown-at-compile time.

[Tokazama]

I'm a little concerned about ambiguity of dynamic referring to changing size vs size-unknown-at-compile time.

I did bring up having a distinction between static, fixed, and dynamic sizes in the referenced issue but I didn't include these because something like UnitRange couldn't have a dynamic size even if it's not known at compile time. I'd be happy to include them if it's seen as a necessary inclusion.

is_dynamic_sized ?

Although that's more specific I'm not sure what else would be "dynamic", so I prefer the is_dynamic syntax. However, I'd be open to changing if people feel strongly.

[ChrisRackauckas]

can_change_size

[Tokazama]

can_change_size seems like it would be much more consistent with the other methods here. I'll change it.