allow full instrumentation of internal operations

[jaypipes]

In order to determine where sqltoast is spending its time during tokenization and parsing efforts, I'd like to provide full instrumentation of all primary functions. The instrumentation "interface" should look something like this:

#include <iostream>
#include <cctype>

#include <sqltoast.h>
sqltoast::parse_input_t subject("CREATE SCHEMA s1");
sqltoast::parse_options_t opts;
opts.enable_instrumentation = true;

auto res = sqltoast::parse(subject, opts);
for (auto instr = res.instrumentation) {
    std::cout << instr.block << ": " << instr.total_ns << " nanoseconds" << std::endl;    
}

which would output something like this:

tokenize(): 31880 nanoseconds
 -> token_comment(): 678 nanoseconds
 -> token_keyword(): 23449 nanoseconds
 -> token_identifier(): 6777 nanoseconds
parse_statement(): 1156 nanoseconds
 -> parse_create_schema(): 928 nanoseconds