Rewrite prepared query parameter substitution.
Query rewrite code

Unify all query rewrite code.
Break all commands down into indivual queries.
NpgsqlParse now takes a byte[] query string.
Use a more efficient method to get at the individual queries in a command.
Add speed test for Prepare().
Cleanup

Rename text => commandText, type => commandType.
Move parameter name character lookup construction to its own function.
Split NpgsqlCommand.cs into bite-size pieces.
NpgsqlCommand.Rewrite

Avoid scanning a command string if possible when no parameters present.
Add an ASCIIByteArrays value for cr/lf line ending.