Parsing problems with \def with templates

Currently, a template can match in the middle of a control sequence name.  For example

```
\def\x#1b#2.{\texttt{#1}}
\x\abc.
```

will output `\a` since the `b` matches in the middle of the `\abc`.

Also, the code

```
\def\x#1\a#2.{\texttt{[#1][#2]}}
\x abc \abc abc \a def .
```

will output `[abc ][bc abc \a def]` rather than the expected `[abc \abc abc ][ def ]`.
