ref(wasm-split): Simplify varint encoding prefix

[bkotsopoulossc]

#skip-changelog

Follow-up to #619.

I wasn't quite sure at the time how to reuse the LEB128 / varint encoding prefix from the wasmbin library. It turns out that it is easy to do with the Encode trait.

The library has some basic encoding support for builtin types. Strings get encoded as binary contents, and binary contents (RawBlob) automatically get encoded with the length before the contents, and the length integer gets encoded as a LEB128 / varint.

Tested by comparing the emscripten output with the wasm-split flag.

% emcc -gseparate-dwarf=foobar bazel/hello-world/hello-world.cc -o test.wasm
% wasm-objdump -j external_debug_info -s test.wasm

test.wasm:	file format wasm 0x1

Contents of section Custom:
0050c3c: 1365 7874 6572 6e61 6c5f 6465 6275 675f  .external_debug_
0050c4c: 696e 666f 0666 6f6f 6261 72              info.foobar

% cargo build --release --package wasm-split 
% ./target/release/wasm-split --strip-names --strip test.debug.wasm -o test.wasm -d test.symbols.wasm --external-dwarf-url=foobar
% wasm-objdump -j external_debug_info -s test.wasm

test.wasm:	file format wasm 0x1

Contents of section Custom:
0957a13: 1365 7874 6572 6e61 6c5f 6465 6275 675f  .external_debug_
0957a23: 696e 666f 0666 6f6f 6261 72              info.foobar

PTAL @Swatinem

[Swatinem]

One less dependency is always a good thing ;-)

[Swatinem]

The failing unit test is again a test missing credentials, so I will just ignore that and merge.