Refactor spring-boot-loader to work directly with low level zip data
structures, removing the need to read every byte when the application
loads.

This change was initially driven by the desire to improve tab-completion
time when working with the Spring CLI tool. Local tests show CLI
startup time improving from ~0.7 to ~0.22 seconds.

Startup times for regular Spring Boot applications are also improved,
for example, the tomcat sample application now starts 0.5 seconds
faster.