Register macros have incorrect hygiene

[odow]

You cannot assume that symbols are in scope in the macros. The fix is the interpolate them in various places, so $ExaModels and $Constant.

julia> import ExaModels as EM

julia> f(x) = exp(x)
f (generic function with 1 method)

julia> EM.@register_univariate(f, f, f)
ERROR: UndefVarError: `ExaModels` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: ExaModels is loaded but not imported in the active module Main.
Stacktrace:
 [1] top-level scope
   @ ~/.julia/packages/ExaModels/KqkPA/src/register.jl:59

julia> import ExaModels

julia> EM.@register_univariate(f, f, f)
ERROR: UndefVarError: `Constant` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: a global variable of this name also exists in ExaModels.
Stacktrace:
 [1] top-level scope
   @ ~/.julia/packages/ExaModels/KqkPA/src/register.jl:63

julia> using ExaModels: Constant

julia> EM.@register_univariate(f, f, f)