Correct the mutability of arguments to RUST_js_GetErrorMessage.
Merge pull request #88 from Ms2ger/js_GetErrorMessage

Correct the mutability of arguments to RUST_js_GetErrorMessage.
Add definitions for the JSREPORT_* constants.
Merge pull request #89 from Ms2ger/JSREPORT

Add definitions for the JSREPORT_* constants.
Add JSVal definitions for 32bit platforms.