Adds ChainRulesCore.rrule for constructors and getproperty

[torfjelde]

This PR adds AD rules for:

• LArray{syms}
• SLArray{size,syms}
• getproperty

This all the issues in #91 with the exception of the ForwardDiff.jl-issue for SLArray.

Some things of note/questions to address:

• Are there more constructors we need to add implementations for? There are quite a few constructors, but it's a bit unclear which we need custom rules for atm. For example it might be nice to have rules defined for LVector(::NamedTuple), which currently does not work.

[ChrisRackauckas]

Awesome thanks!