feat(xsnap): Base64 C bindings'

[kriskowal]

• feat(xsnap): Update xsnap-native for base64
• feat(xsnap): Add Base64 C bindings

refs: #2682

Description

This change adds a Base64 namespace with encode and decode functions.
A pure JavaScript Base 64 codec performs very poorly on XS.
Decoding 400KB is a common load for a Base 64 encoded Agoric zip archive source bundle,
and takes about an hour to decode in xsnap.
This change surfaces XSnap bindings for Base 64 based on Moddables existing module.

agoric-labs/xsnap-pub#4

Security Considerations

XSnap does not harden the Base64 namespace object.
It should be hardened before introduction to a compartment as an endowment.

Documentation Considerations

This should be transparent to users.
Subsequent work in @endo/base64 will harness this API if it is present on the
global object and work around one known defect by having a fast path for empty
input.

Testing Considerations

This change adds the new Base64 and existing TextEncoder and TextDecoder to the
XSnap REPL so reviewers can fiddle with it using yarn repl in the xsnap
package.

This includes a failing test, where an empty command buffer, Base64 encoded,
then UTF-8 encoded, effects a detached buffer error.
We can work-around this in @endo/base64 for now.

[dckc]

security considerations += expands the scope of the upcoming memory safety audit (#2466).

@jessysaurusrex In that scoping meeting, I said we use only the xs interpreter code and not the modules. But we started pulling in modules with #3797 . We plan to use sha512 (#3512 ) as well.

[dckc]

for the other FFI bindings, we test that they are only available in the start compartment.

When adding Base64, we should probably refactor those tests to just handle a list of things.

[dckc]

nit: for the 2 submodule updates, I suggest build rather than feat, following cosmos-sdk conventional-commit-types where build is defined as "Changes that affect ... external dependencies".

Only "Add Base64 C bindings" is a feat that I would expect to see in the xsnap release notes.