src: general C++ cleanup in node_url.cc

• Merge domain and opaque storage in URL parser:

  This just simplifies the code a bit, having multiple fields in an union with the same type is usually just overhead.

• Add move variant of URLHost::ToString():

  This helps avoid unnecessary string copy operations, especially since we control the lifetime of URLHost objects pretty well.

• Use const refs in node_url.cc where appropriate

• Remove or reduce overly generous .reserve() calls:

  These would otherwise keep a lot of unused memory lying around.

• Return return values instead of unnecessary pointer arguments

• Use more common/expressive variable names

• Avoid macro use, reduce number of unnecessary JS strings:

  There’s no reason for GET, GET_AND_SET and UTF8STRING to be macros. Also, GET would previously create a JS string instance for each single call, even though the strings it was called with were compile-time constants.

• Avoid unnecessary JS casts when the type of a value is known

• Avoid (commonly unnecessary) copy for whitespace stripping

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines