check for zero-length function names
allow setting breakpoints in functions containing raw NA
make user code detection more symmetric with call stack generation
fix exception when listing environment during non-top-level but non-debug sessions
Merge pull request #57 from rstudio/feature/environment-grid-view

Fix 'file_name' exception when listing environment in certain settings
Merge remote-tracking branch 'origin/master' into bugs/debugging
don't enter debug mode during init if there's no active browser
gracefully handle case where code model has no method for significant token seeking
Merge pull request #58 from rstudio/bugs/debugging

Fix misc bugs in debugging features