Add constant folding.

## Overview

This pull request adds constant folding which collapses constant expressions (including `now()`) into a simplified expression. This is a useful optimization for minimizing numeric binary expression operations, however, more importantly it simplifies the handling of time range expressions such as:

```
SELECT count() FROM cpu WHERE time > now() - 1h
```

This query will get folded into a simpler expression at execution time:

```
# Assuming current time is Jan 1, 2000 00:00:00
SELECT count() FROM cpu WHERE time > "2000-01-01 00:00:00"
```

See line notes for more details.

## Supported Types

The following binary expression combinations can be folded into a constant:

```
NUMBER + NUMBER     (arithmetic, comparative)
BOOLEAN + BOOLEAN   (equality, logical)
TIME + TIME         (subtraction, comparative)
TIME + DURATION     (addition, subtraction)
DURATION + DURATION (addition, subtraction, comparative)
DURATION + NUMBER   (multiplication, division)
STRING + STRING     (concatenation)
```

Also, the `now()` function call is evaluated to the current time at the beginning of query execution. This is to ensure that the time does not skew during the course of execution.

Yep. There wasn't a test with a `VarRef` with a prefix so it wasn't tested before.