This needs to run in the browser too, where we have no control over stack
limits.
* build-aux/bootstrap/birth.scm
(lex): Non-recursive strategy (loop with mutable list).
(make-token): Update doc. Produce list of token, new string, and
position. Don't recurse.
(body->es): Add `ret' param. Only produce `return' statement if new param
is set.
(cdfn): Use it.
(fnmap)
[js:while, js:break]: Add forms.
[lambda, let, case]: Use new `body->es' `ret' param.
[let*]: Define JS variables in output using `let' instead of `const' to
permit mutating with new `set!' form. Use new `body->es' `ret' param.
[set!]: Add form.
(prebirth->ecmascript): Adjust libprebirth path to be relative to self.
* build-aux/bootstrap/libprebirth.js
($$append$b$): Add `append!' procedure.
($$js$regexp, $$js$match, $$js$replace): Move a few lines up.
(fs): Provide stub if `require' is not defined.
* build-aux/bootstrap/prebirth.js
(_lex): Non-recursive strategy (loop with array-appending).
(_token): No more mutual recursion with `#_lex'. Return new string
and position.
(_bodyToEs): Add `ret' param. Only produce `return' statement if new
param is set.
(fnmap) [js:while, js:break]: Add forms.
[let*]: Define JS variables in output using `let' instead of `const' to
permit mutating with new `set!' form. Use new `body->es' `ret' param.
[set!]: Add form.