switch to cpp11 and fix enloc bugs

[danielnachun]

This PR is a major update with several important changes:

• I have switched from Rcpp to cpp11. cpp11 is a more modern interface for C++ in R developed by the tidyverse team. It nicely cleans up our code and should make compilation easier
• I verified our C++ ports with the upstream code using Claude and nothing needed to be changed except for a few small bugs with the enloc QTL enrichment
• We now have proper test fixtures for our basic genotype handling functions
• These test functions have now allowed us to make all of the genotype handling functions fully generic across .bed/.bim/.fam, .pgen/.pvar/.psam, VCF and GDS.

The only test fixtures I think we need to add are:

• TWAS weights
• multitrait summary statistics RDS (for mashr)
• susie fit RDS
• tensorQTL summary statistics
• stochastic genotypes

[codecov]

Codecov Report

❌ Patch coverage is 82.01439% with 100 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.34%. Comparing base (0517999) to head (6b9fc06).
⚠️ Report is 1041 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dentist_iterative_impute.cpp	41.53%	38 Missing ⚠️
src/qtl_enrichment.hpp	67.41%	29 Missing ⚠️
R/file_utils.R	90.31%	25 Missing ⚠️
R/LD.R	88.00%	3 Missing ⚠️
R/mash_wrapper.R	0.00%	2 Missing ⚠️
src/sdpr_mcmc.cpp	66.66%	2 Missing ⚠️
src/lassosum_rss.cpp	96.55%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #474       +/-   ##
===========================================
+ Coverage   27.48%   88.34%   +60.85%     
===========================================
  Files          32       35        +3     
  Lines        4377     6906     +2529     
===========================================
+ Hits         1203     6101     +4898     
+ Misses       3174      805     -2369     

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