Reduce repeated calls to node name and type

[transoceanic2000]

Initial attempt to help with #122

In the main loop we may call node.name and node.node_type several times in different if statement. node.name in particular results in allocation of a string, boosting memory usage. Calling them once and storing them locally reduces memory use.

In a test reading 10k (mostly empty) rows from a spreadsheet can see that the extra allocations from calling node.name in later branches of the if disappear from the stats. Overall %age reduction in allocations is small (~2% in this case), so won't fix the memory usage but will help. %age reduction may be larger for sheets that contain more cells, as the later branches will be exercised more often.

sourcefile	sourceline	class	count	comments
gems/nokogiri-1.18.2-arm64-darwin/lib/nokogiri/xml/reader.rb	127	String	64407
gems/rubyzip-2.4.1/lib/zip/inflater.rb	19	String	42986
gems/nokogiri-1.18.2-arm64-darwin/lib/nokogiri/xml/reader.rb	127	Hash	33405
gems/rubyzip-2.4.1/lib/zip/ioextras/abstract_input_stream.rb	20	String	28666
gems/rubyzip-2.4.1/lib/zip/inflater.rb	19	Range	14330
creek/lib/creek/sheet.rb	122	String	11565	remains the same
creek/lib/creek/sheet.rb	110	Hash	11565
creek/lib/creek/sheet.rb	125	Hash	10001
creek/lib/creek/sheet.rb	124	Hash	10001
creek/lib/creek/sheet.rb	127	String	1564	removed
creek/lib/creek/sheet.rb	145	String	1562	removed
creek/lib/creek/sheet.rb	141	String	1537	removed
creek/lib/creek/sheet.rb	148	String	706