Update type definitions of random matrix ensembles

[apkille]

The type definitions for Ginibre, GaussianHermite, GaussianJacobi, Ginibre, and Haar are wonky, and they could all become parametric immutable types instead. Below, I list their definitions and suggest improvements, some of which are breaking:

RandomMatrices.jl/src/GaussianEnsembles.jl

Lines 43 to 44 in fc98f3b

	struct GaussianHermite{β} <: ContinuousMatrixDistribution end
	GaussianHermite(β) = GaussianHermite{β}()

struct GaussianHermite{B} <: ContinuousMatrixDistribution 
    beta::B
end 

RandomMatrices.jl/src/GaussianEnsembles.jl

Lines 175 to 179 in fc98f3b

	mutable struct GaussianLaguerre <: ContinuousMatrixDistribution
	beta::Real
	a::Real
	end
	const Wishart = GaussianLaguerre

struct GaussianLaguerre{B,A} <: ContinuousMatrixDistribution 
    beta::B
    a::A
end 

RandomMatrices.jl/src/GaussianEnsembles.jl

Lines 277 to 282 in fc98f3b

	mutable struct GaussianJacobi <: ContinuousMatrixDistribution
	beta::Real
	a::Real
	b::Real
	end
	const MANOVA = GaussianJacobi

struct GaussianJacobi{B,A} <: ContinuousMatrixDistribution 
    beta::B
    a::A
    b::A
end 

RandomMatrices.jl/src/Ginibre.jl

Lines 27 to 30 in fc98f3b

	struct Ginibre <: ContinuousMatrixDistribution
	beta::Float64
	N::Integer
	end

struct Ginibre{B} <: ContinuousMatrixDistribution 
    beta::B
end 

RandomMatrices.jl/src/Haar.jl

Lines 91 to 93 in fc98f3b

	mutable struct Haar <: ContinuousMatrixDistribution
	beta::Real
	end

struct Haar{B} <: ContinuousMatrixDistribution 
    beta::B
end 

@dlfivefifty please let me know your thoughts, I'm happy to implement these changes. I'm also aware that I'm new to this old package, so I won't be upset if you turn down this proposal 😄