Best effort assert printing

[erights]

Fixes #729

@dckc I should still take your suggestion to give detailsTokens their own toString method. However, that solves less of the problem. For example, the test case added in this PR would still fail. Instead, where the assert module itself stringifies something directly, not through details, it would now stringify the quote of it. IOW, it would use bestEffortStringify.

Since I don't yet know how to reproduce #729, I'll start with this PR, and take your suggestion later after we see if this PR solves your problem.

[dckc]

I can check whether this addresses the specific case that I reported, but ...

the whole agoric-sdk seems to assume that anything can be stringified. For example, the `${s}` idiom for coercing unknown to string -- I don't think it's expected to throw. While it's evidently not 100% true that everything can be stringified, I think we're better off not purposefully creating objects that throw on `${obj}`.

[dckc]

Meanwhile, to reproduce the problem:

build xst

git clone https://github.com/Moddable-OpenSource/moddable
cd moddable
export MODDABLE=$PWD
export PATH="${MODDABLE}/build/bin/mac/release:$PATH"
cd xs/makefiles/mac
make

run a 1-liner

$ xst -e 'print(`${{ __proto__: null }}`)'
TypeError: ?: cannot coerce object to string

[erights]

@dckc this is now ready for review. PTAL

[dckc]

good stuff.

I made one editorial suggestion for you to consider.