WIP: perf(theme): optimize theme handling

[levithomason]

Merging themes on context is a hot code path. It is also heavy due to merging themes and applying variables.

This PR is experimenting with approaches to optimize this path for all components.

Initial results are promising, just memoizing mergeThemes produces 75-95% performance gain in all components:

==============================
Button.perf.tsx
BEFORE  AFTER  SAVED  
42.83   2.89   93%    
43.53   1.57   96%    
10.57   1.11   89%    
86.06   19.54  77%    

==============================
Header.perf.tsx
BEFORE  AFTER  SAVED  
1.13    1.3    -15%   
0.69    0.66   4%     
0.45    0.46   -2%    
4.71    7.63   -62%   

==============================
Icon.perf.tsx
BEFORE  AFTER  SAVED  
8.34    1.33   84%    
8.69    0.56   94%    
3.27    0.38   88%    
16.25   7.99   51%    

==============================
Loader.perf.tsx
BEFORE  AFTER  SAVED  
25.84   1.86   93%    
31.44   1.14   96%    
5.93    0.82   86%    
48.51   11.24  77%    

==============================
HeaderDescription.perf.tsx
BEFORE  AFTER  SAVED  
7.35    2.25   69%    
5.86    1.3    78%    
2.53    0.89   65%    
15.79   16.41  -4%    

==============================
Attachment.perf.tsx
BEFORE  AFTER  SAVED  
35.33   2.47   93%    
26.54   1.44   95%    
7.94    1.04   87%    
68.63   11.54  83%    

==============================
Divider.perf.tsx
BEFORE  AFTER  SAVED  
8.26    1.18   86%    
8.42    0.58   93%    
3.27    0.41   87%    
16.49   11.22  32%    

==============================
List.perf.tsx
BEFORE  AFTER  SAVED  
68.27   68.78  -1%    
67.67   65.39  3%     
54.4    56.3   -3%    
98.21   99.29  -1%    

==============================
Chat.perf.tsx
BEFORE  AFTER  SAVED  
284.64  40.03  86%    
276.29  39.44  86%    
208.04  30.11  86%    
426.62  82.98  81% 

[layershifter]

I can be wrong, but Lodash memoizes only the first param, lodash/lodash#2115. fast-memoize works with multiple arguments.