Making sexp_env_cell_define available.
Factoring out generate opcode and exporting it.
Adding various environment and core AST accessors needed for the pure-scheme eval.
temporarily disabling scheme-report-environment test
Using scheme load when available for scripts to preserve stack traces.
Using non-mutating tree of dynamic-wind state for thread safety.
Adding note about the derived syntax definition of quote.  It makes
compilation slightly but noticeably slower and is inconvenient, so
leaving as a core form for now.
exact-integer-sqrt should continue the babylonian method in scheme.
The estimate given by the C code could be off by a large margin when
the result is inexact, converging far too slowly if we use simple
increments in Scheme.
Adding a sanity check in sexp_bignum_sqrt.
Need to GC preserve earlier in sexp_sqrt.