Don't allocate z or m members of an xy unless they're needed

[paleolimbot]

Fixes #131.

Before:

library(wk)

many_points <- as_wkb(xy(runif(1e6), runif(1e6)))
bench::mark(as_xy(many_points))
#> # A tibble: 1 × 6
#>   expression              min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>         <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 as_xy(many_points)     67ms   67.3ms      14.9    30.6MB     37.2

^{Created on 2022-09-21 by the reprex package (v2.0.1)}

After:

library(wk)

many_points <- as_wkb(xy(runif(1e6), runif(1e6)))
bench::mark(as_xy(many_points))
#> # A tibble: 1 × 6
#>   expression              min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>         <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 as_xy(many_points)   24.6ms   25.1ms      39.8    15.3MB     62.5

^{Created on 2022-09-21 by the reprex package (v2.0.1)}

[codecov-commenter]

Codecov Report

Base: 98.95% // Head: 98.92% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (7deaaa8) compared to base (d43c2c2).
Patch coverage: 94.87% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   98.95%   98.92%   -0.04%     
==========================================
  Files          74       74              
  Lines        5173     5198      +25     
==========================================
+ Hits         5119     5142      +23     
- Misses         54       56       +2     

Impacted Files	Coverage Δ
src/xy-writer.c	98.90% <94.87%> (-1.10%)	⬇️

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