Replace log with tracing, instrument conversion functions

[jonmmease]

Incorporates the tracing crate to instrument the conversion functions. For example, from Python:

import os
os.environ["RUST_LOG"]="vl_convert"

...
png = vlc.vega_to_png(vg_spec)

2024-02-03T18:52:42.500059Z  INFO vega_to_png:vega_to_svg: vl_convert_rs::converter: close time.busy=18.2µs time.idle=237ms
2024-02-03T18:52:42.510416Z  INFO vega_to_png:svg_to_png: vl_convert_rs::converter: close time.busy=10.3ms time.idle=1.54µs
2024-02-03T18:52:42.510432Z  INFO vega_to_png: vl_convert_rs::converter: close time.busy=10.4ms time.idle=237ms