Implement memoization in cfkey.

This patch fixes issues #26.
Fix cannot locate function 'compile_filter'.

* Renamed script 'pc_pcap_asm' to 'bpf-to-asm'.
* Added usage information to 'bpf-to-asm'.
* Move script to dir 'utils/'.
Merge pull request #33 from Igalia/cannot-locate-function-compile_filter

Fix cannot locate function 'compile_filter'.
Merge pull request #26 from Igalia/issue-26-fix-cfkey

Fix n² complexity in cfkey
Implement symbolic constants.

- Symbolic constants in arithmetic expressions. Add support for
constansts: 'protocol header field offsets', 'icmp type field values'
and 'tcp flag field values'.

- Symbolic constants in logical operations. Add support for constants
names for port numbers. Valid in 'port <port>' and 'portrange <port-port>'
contexts.

This patch solves issue #24.