Fix pretty printing of polynomials (mostly)

[jakewilliami]

1. Made unicode super- and subscripts more compact

2. Performance considerations using print(io, ...) instead of join.

3. Code cleanup. Merged the print_poly function directly into Base.show, as print_poly was only being used in the show definition (i.e., the function was redundant).

4. The pretty_var function was not printing correctly, causing tests to fail—the reason for this is because polynomials would sometimes print in the form

2z₂⁴

But sometimes of the form

2z[2]⁴

I'm not sure what caused that, nor for how long the tests were broken, but I just added an extra regex matching in the pretty_var function to capture this. The only thing failing now is converting variables of Polynomial to Symbols correctly:

poly: Test Failed at /home/jakewilliami/projects/FixedPolynomials.jl/test/poly_test.jl:41
  Expression: variables(F[1]) == [:z1, :z2]
   Evaluated: [Symbol("z[1]"), Symbol("z[2]")] == [:z1, :z2] 
# ...
Test Summary: | Pass  Fail  Total
poly          |   50     1     51
ERROR: LoadError: LoadError: Some tests did not pass: 50 passed, 1 failed, 0 errored, 0 broken.

Sorry I haven't been able to fix the Symbol problem. It would be good to know what changes caused Polynomials to print in 2 different ways.

[jakewilliami]

By the way, it would be nice to have a Project.toml file I think 🙂 This will replace REQUIRE.

[jakewilliami]

Another small performance consideration that I can't do right now, but you could consider using view where possible (i.e., in the calculation of degree:

julia> @btime sum(exponents(p)[:, 1]) # current implementation of `degree`
  41.922 ns (1 allocation: 112 bytes)
4

julia> @btime @inbounds sum(view(exponents(p), :, 1)) # `degree` using view
  15.358 ns (0 allocations: 0 bytes)
4