We were indiscriminately retrieving the node's provenance data in the
`node_get()` function which was accounting for approximately a third of
the total runtime of `node_get()` which dominates pre-scheduler time in
`bst build`.  This change ameliorates that situation by only retrieving
the provenance data when it's actually needed.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

We know that nodes are typically one of:
str, list, dict, bool, tuple, NoneType or our ChainMap
Of these, dict and ChainMap are Mapping, only list is list
and the rest are returned unchanged.  We can reduce/defer our use
of isinstance here, dramatically, improving performance.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

A non-trivial proportion of time pre-scheduler in `bst build` is spent
copying (or chain-copying) nodes.  Approximately a quarter of the time
spent in that effort is in `isinstance()`.  This removes that CPU load.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

By re-using the isinstance replacements from earlier commits and
using a tuple of the string constants for checking for composition
markers, we reduce the cost of node_final_assertions by two thirds
in basic testing.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

While get_strict() doesn't look expensive per-se, it is called so many
times that it is valuable to cache the result once computed.  Since I
don't think it can change once it is computable, cache it immediately
that becomes possible and we save 20s in my test case.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Further optimisations

See merge request !1131