feat: env var to control libxml2 memory management

[flavorjones]

What problem is this PR intended to solve?

This allows a user to opt into configuring libxml2 to use system malloc instead of ruby's ruby_xmalloc (which is the default on all platforms except windows using system DLLs).

This addresses a few problems:

• it's a workaround for environments affected by the Ruby bug described in [bug] Uptick of seg faults in Nokogiri v1.14.0 #2785 and fixed in Ensure ruby_xfree won't segfault if called after ruby_vm_destruct ruby/ruby#7663
• it improves parsing performance by ~10-30%, and serialization performance by ~5-10%

Parsing benchmarks:

default memory html4 parse 656:    27145.8 i/s
   ruby memory html4 parse 656:    24287.1 i/s - 1.12x  (± 0.04) slower

default memory html4 parse 70095:      605.9 i/s
   ruby memory html4 parse 70095:      450.5 i/s - 1.34x  (± 0.13) slower

default memory html4 parse 681406:       73.7 i/s
   ruby memory html4 parse 681406:       56.2 i/s - 1.31x  (± 0.09) slower

default memory html4 parse 1929522:       52.3 i/s
   ruby memory html4 parse 1929522:       43.7 i/s - 1.20x  (± 0.06) slower

Serialization benchmarks:

default memory html4 srlze 656:    78223.0 i/s
   ruby memory html4 srlze 656:    77631.2 i/s - same-ish: difference falls within error

default memory html4 srlze 70095:     1749.3 i/s
   ruby memory html4 srlze 70095:     1619.7 i/s - 1.08x  (± 0.01) slower

default memory html4 srlze 681406:      179.8 i/s
   ruby memory html4 srlze 681406:      162.1 i/s - 1.11x  (± 0.01) slower

default memory html4 srlze 1929522:      184.1 i/s
   ruby memory html4 srlze 1929522:      172.9 i/s - 1.06x  (± 0.01) slower

I'm not making this the default right now -- though we should consider doing so in the future -- because in production environments, it may change the memory usage behavior.

Have you included adequate test coverage?

Yes!

Does this change affect the behavior of either the C or the Java implementations?

No functional changes.

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cc @byroot and @tenderlove