Path: optimize join by precomputing capacity if possible

[asterite]

Nothing spectacular, but still...

Code:

require "benchmark"

base = "/Users/someone/foo/bar"
parts = ["baz", "qux", "foo", "bar", "baz"]

Benchmark.ips do |x|
  x.report("join") do
    Path.new(base).join(parts)
  end
  x.report("join (new)") do
    Path.new(base).join2(parts)
  end
end

base = "/Users/someone/foo/bar"
parts = ["baz.cr"]

Benchmark.ips do |x|
  x.report("join") do
    Path.new(base).join(parts)
  end
  x.report("join (new)") do
    Path.new(base).join2(parts)
  end
end

Results:

      join   5.28M (189.53ns) (± 3.96%)  176B/op   1.05× slower
join (new)   5.52M (181.08ns) (± 1.03%)  160B/op        fastest
      join   8.64M (115.69ns) (± 3.00%)  176B/op   1.22× slower
join (new)  10.57M ( 94.59ns) (± 3.49%)  144B/op        fastest

Path#join also has a bug: in one part of the code it calls parts.size but it's not fine to assume that given that it's just Enumerable. But it's probably fine because at least in the std all enumerables have a known size.