Declare global names as const

[jishnub]

This helps with type inference.

On master

julia> d = ChebyshevInterval();

julia> f(d) = DomainSets.:(×)(d, d)
f (generic function with 1 method)

julia> @code_warntype f(d)
MethodInstance for f(::ChebyshevInterval{Float64})
  from f(d) @ Main REPL[5]:1
Arguments
  #self#::Core.Const(Main.f)
  d::Core.Const(-1.0 .. 1.0 (Chebyshev))
Body::Any
1 ─ %1 = Main.DomainSets::Core.Const(DomainSets)
│   %2 = Base.getproperty(%1, :×)::Any
│   %3 = (%2)(d, d)::Any
└──      return %3

with this pR

julia> @code_warntype f(d)
MethodInstance for f(::ChebyshevInterval{Float64})
  from f(d) @ Main REPL[3]:1
Arguments
  #self#::Core.Const(Main.f)
  d::Core.Const(-1.0 .. 1.0 (Chebyshev))
Body::DomainSets.FixedIntervalProduct{2, Float64, ChebyshevInterval{Float64}}
1 ─ %1 = Main.DomainSets::Core.Const(DomainSets)
│   %2 = Base.getproperty(%1, :×)::Core.Const(DomainSets.cartesianproduct)
│   %3 = (%2)(d, d)::Core.Const((-1.0 .. 1.0 (Chebyshev)) × (-1.0 .. 1.0 (Chebyshev)))
└──      return %3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.86%. Comparing base (905e0b6) to head (8244b74).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   93.86%   93.86%           
=======================================
  Files          27       27           
  Lines        1843     1843           
=======================================
  Hits         1730     1730           
  Misses        113      113           

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