tp: bundle source files and disassembly alongside symbols

`trace_processor bundle` now also writes sources.pb and disassembly.pb
members so the UI can show annotated source and assembly without the
binaries or source tree at hand.

Source files are the ones referenced by symbolized frames, both from
symbols already in the trace and those just produced. Only absolute
paths are tried, files over 1 MB are skipped and at most 16 MB is
bundled; --source-prefix-map FROM=TO reads files under a different root
while keeping the debug info path as the key, and --no-sources skips
the step.

Disassembly covers the functions containing sampled addresses of every
mapping the local symbolizer resolved against a binary on disk. The
symbolizer now reports that binary and the mapping-to-link-time address
correction; the bundler finds the containing functions with llvm-nm and
disassembles each with llvm-objdump, keeping bytes, text, statically
known branch targets and source lines. When the symbols came from a
split debug file (a .dSYM bundle or an ELF holding only debug info) the
code is taken from the mapping's own path, verified to have the same
build id, with objdump pointed at the debug file for lines. The last
symbol of a section extends to the section's end. --no-disassembly
skips the step. Both steps report what they bundled and, with
--verbose, what they could not.