Fuzz improvements

[graydon]

Significant improvements to soroban's wasm-based fuzzing:

• Actually retrieves passed-in arguments correctly
• Performs rudimentary cross-contract (vm-to-vm) calls
• Records and then enforces footprints and auth, so:
  • Gets into storage system
  • Gets into auth system
• Records and summarizes coverage information
• Has a deterministic baseline it can compare coverage to to see if it broke

Still needs a little work (linear memory, weird sections) but will wrap that up tomorrow.

Fixes #1136

lcov-summary fuzz-lcov-baseline.info fuzz-lcov.info
                                    Lines                  Functions      
                          │  Hit   Total  H/T      │  Hit  Total  H/T 
 fuzz-lcov-baseline.info  │  6183  14611   42.32%  │  846   2004   42.22% 
          fuzz-lcov.info  │  6183  14611   42.32%  │  846   2004   42.22% 
                    diff  │               = 0.00%  │              = 0.00%