Revert "[vm] Have Read/WriteStream use (S)LEB128 for variable-length encoding."

This reverts commit cfc8e6d.

Reason for revert: b/167520550

Original change's description:
> [vm] Have Read/WriteStream use (S)LEB128 for variable-length encoding.
>
> Previously, the non-fixed read and write methods of ReadStream and
> WriteStream used a similar but slightly different encoding to LEB128 for
> unsigned values and SLEB128 for signed values. The only difference was
> the high continuation marker bit, which was unset in non-final parts and
> set in final parts in the previous encoding, but is set for non-final
> parts and unset in final parts in (S)LEB128.
>
> This CL changes it so that they instead use the standard LEB128 and
> SLEB128 encodings. Among other things, this means that one-byte
> encodings of values in snapshots will be the value itself, not the value
> with the high bit set. This also means that when outputting specific
> formats that use (S)LEB128, like DWARF, the format writers can just use
> WriteStream::Write(Unsigned) directly instead of needing to write a
> separate encoder.
>
> Cq-Include-Trybots: luci.dart.try:vm-kernel-ubsan-linux-release-x64-try,vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-linux-product-x64-try,vm-kernel-precomp-linux-debug-x64-try,vm-kernel-precomp-linux-release-simarm_x64-try
> Change-Id: I409a22e7da9e4672011dfaccd3790c122ec3e522
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150526
> Commit-Queue: Tess Strickland <[email protected]>
> Reviewed-by: Ryan Macnak <[email protected]>

[email protected],[email protected],[email protected]

Change-Id: Ie5b9e595e4dbe9bbed5f0dd5bdc6f78bd1233f5d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161540
Commit-Queue: Tess Strickland <[email protected]>
Reviewed-by: Tess Strickland <[email protected]>

Author: sstrickl