Use Defined for EqDec instances instead of Qed

[jbaum98]

As described in PrincetonUniversity/VST#290, the use of Qed when creating typeclass instances makes it so that you can't evaluate expressions using the typeclass:

Compute 1 ?= 1.
     = (let (eqb, _) := nat_EqDec in eqb) 1 1

The instances in EqDec.v that currently end in Qed and shoulde be changed to Defined are:

• Vector_EqDec
• unit_EqDec
• bool_EqDec
• nat_EqDec
• option_EqDec
• list_EqDec

[adampetcher]

This is likely a good idea, but making definitions transparent tends to cause problems when they are used. At least, it used to cause problems in older versions of Coq. Can you produce a pull request so I can see how well this works in Coq 8.8?