Avoid StringReader when checking code blocks for syntax errors
Avoid StringReader when checking code blocks for syntax errors

`parse_stream_from_source_str` is a more stable API to convert a
string into a bunch of tokens, and it also catches errors about
mismatched parenthesis.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust