0000000d-000a-0000-0000-000000000000 not recognized as UUID by lexer

[taskwiki](https://github.com/tbabej/taskwiki) tests uncovered that some UUID aren't parsed as UUIDs:

```
$ task 0003717d-b325-4fd1-8367-7492996250df export
Cannot subtract strings
$ task 0000000d-000a-0000-0000-000000000000 export
Cannot subtract strings
$ task 0000000d-0000-0000-0000-000000000000 export
The expression could not be evaluated.
```

I've created a failing test PR to demonstrate the problem: https://github.com/GothenburgBitFactory/taskwarrior/pull/2305, https://travis-ci.org/github/GothenburgBitFactory/taskwarrior/builds/705700719
Failing test: https://travis-ci.org/github/GothenburgBitFactory/taskwarrior/jobs/705700721#L6223

I've also found a workaround: `task uuid:0000000d-000a-0000-0000-000000000000 export` works correctly, but I'm not sure if this workaround can be used everywhere one may need to use an UUID. It seems to work for setting dependencies.

```
$ task diag

task 2.5.1
Platform: Linux

Compiler
Version: 9.2.1 20200224
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
CMake: 3.16.3
libuuid: libuuid + uuid_unparse_lower
libgnutls: 3.6.12
Build type: None

Configuration
File: /home/tomi/.taskrc (found), 1225 bytes, mode 100644
Data: /home/tomi/taskwiki/.task (found), dir, mode 40755
Locking: Enabled
GC: Enabled
$EDITOR: vim
Server:
Trust: strict
Certificate: , not readable, 0 bytes
Key: , not readable, 0 bytes
Ciphers: NORMAL
Creds:

Hooks
System: Enabled
Location: /home/tomi/taskwiki/.task/hooks
(-none-)

Tests
$TERM: rxvt-unicode-256color (144x42)
Dups: Scanned 166 tasks for duplicate UUIDs:
No duplicates found
Broken ref: Scanned 166 tasks for broken references:
No broken references found

```