Improve column number tracking

[chqrlie]

• simplify column number tracking using a pointer to the beginning of line instead of eol + mark.
• add js_parse_error_pos to report syntax errors with correct column number for token parsing errors. This makes the syntax error reports much more precise.
• add JSSourcePos type to keep track of token source position
• add emit_op_pos to set the precise source position in code generation this is work in progress: it should be the only way to generate source positions and we should get rid of s->last_line_num and s->last_col_num.
• runtime errors on calls report the column number of calling function or method name.
• runtime errors on new expressions report the column number of the neẁ keyword.
• update tests/test_builtin,js with more informative messages
• improve assert() and tests/test_language.js tests
• update v8.txt for updated column numbers in remaining errors

[saghul]

I'll defer this review to @bnoordhuis who worked on adding column numbers :-P