Speed up writing to global variables

[ChrisJefferson]

This PR is based around the observation that when we assign a global variable, we have to check iExprsGVar, FopiesGVar or CopiesGVar for that global variable. However, for most global variables they are set to their default values. This PR adds a single flag which tells us, for each gvar, us if any of these 3 arrays have a non-default value.

To avoid introducing another array, we take WriteGVars (which stores one of three values, if a gvar is mutable, read-only or constant), and make it into FlagsGVars, which now stores this original information, and an extra bit that tells us if any of Exprs,FOpies or Copies is not default. As we have to read WriteGVars anyway, whenever we assign a global, this means there are no extra memory lookups, just the unpacking of the flags.

For simplicity, this new bit is never cleared once it is set. This means when it is true we don't know for certain one of exprsgvar, fopiesgvar or copiesgvar is set to a non-default value, but if it is false we know they are are default.

In a tight loop assigning a global, this PR speeds things up by about 40%

• Release Notes: Speed up writing to global variables

[fingolfin]

great job! Some quick remarks, I'll have a proper look after some sleep

[codecov]

Codecov Report

Merging #3579 into master will increase coverage by 1.57%.
The diff coverage is 98.03%.

@@            Coverage Diff             @@
##           master    #3579      +/-   ##
==========================================
+ Coverage   83.47%   85.05%   +1.57%     
==========================================
  Files         693      650      -43     
  Lines      344698   319856   -24842     
==========================================
- Hits       287749   272049   -15700     
+ Misses      56949    47807    -9142

Impacted Files	Coverage Δ
src/gvars.c	89.94% <98.03%> (+3.38%)	⬆️
lib/random.g	66.66% <0%> (-30.56%)	⬇️
lib/cache.gi	70.76% <0%> (-24.62%)	⬇️
lib/read2.g	85.71% <0%> (-14.29%)	⬇️
lib/read1.g	85.36% <0%> (-13.42%)	⬇️
lib/helpview.gi	28.77% <0%> (-9.98%)	⬇️
lib/random.gd	91.3% <0%> (-8.7%)	⬇️
lib/methsel2.g	42.66% <0%> (-8%)	⬇️
lib/package.gd	93.47% <0%> (-6.53%)	⬇️
grp/perf.gd	94.44% <0%> (-5.56%)	⬇️
... and 277 more

[coveralls]

Coverage decreased (-0.0001%) to 84.301% when pulling e787b0b on ChrisJefferson:speed-gvars into 0b963ef on gap-system:master.

[ChrisJefferson]

I ended up merging the commits in this PR together, because both of them had been changed so that the middle one wasn't actually buildable any more, and I didn't think it was worth fixing.

[fingolfin]

Fine by me (you may or may not want to fix that typo, it's not important)

[fingolfin]

(And by "fine by me" I obviously meant: Very nice improvement, thank you!! Obviously. Ahem. :-)