User defined variables evaluate incorrectly when prepended with a number during evaluation

e.g.

Given:
```
let g:crunch_user_variables = {'pi':3.14159265359}
```
Then following occours. because in the second line the 2 is being prepended infront of the pi constant and not being multiplied like expected.
```
2*pi = 6.283186
2pi = 23.141593  "expected 6.283186"
```