Avoid deepcopy pt. 2

[nossleinad]

Follow up to PR #6

-Implements copy_partition, which falls back to deepcopy but is overloaded for <:DiscretePartition and GaussianPartition in this PR.
-copy_message is implemented as the trivial generalization of copy_partition.
-deepcopy calls are replaced with copy_message calls in internal_message_init!.

Note: It is meant for copy_partition and copy_message to replace deepcopy calls throughout the whole package. A follow up PR will carry out the rest of these replacements.

Example with GaussianPartition:

tree = sim_tree(n = 1_000_000)
@time internal_message_init!(tree, GaussianPartition())

# main
 10.450746 seconds (50.00 M allocations: 3.178 GiB, 19.87% gc time, 0.05% compilation time)

# PR
 1.794678 seconds (20.00 M allocations: 1.032 GiB, 46.68% gc time)

Example with NucleotidePartition:

tree = sim_tree(n = 100_000)
@time internal_message_init!(tree, NucleotidePartition(30))

# main
 1.075276 seconds (4.43 M allocations: 1.081 GiB, 23.23% gc time, 1.02% compilation time)

# PR
 0.416153 seconds (3.20 M allocations: 912.376 MiB, 58.30% gc time)

Example with both:

tree = sim_tree(n = 100_000)
@time internal_message_init!(tree, [GaussianPartition(), NucleotidePartition(30)])

# main
 1.759613 seconds (6.13 M allocations: 1.084 GiB, 13.33% gc time, 0.63% compilation time)

# PR
0.621201 seconds (5.91 M allocations: 971.212 MiB, 41.53% gc time, 9.70% compilation time)