Improve OpenQASM 3 exporter performance for hardware-like circuits

This reorganises the variable lookups and internal AST generation
slightly to improve the OpenQASM 3 exporter performance for the common
case of circuits with many singly parametric 1q gates, parametrised by a
single `Parameter` expression.  This is quite common in high-throughput
cases, where the circuit is being used as a parametric template for
a backend compiler.