Refactor API

[epage]

This is a major step towards #95

• ParserBuilder -> Parser
  • Takes in includes, tags, blocks, and filters
• Parser -> Template
  • Takes in str
• Template -> String
  • Takes in globals
• Separate concerns
  • Top-level Parser, Template
  • Value
  • Interpreter
  • Syntax / Compile to interpreted state

This also saw a consolidation of how variables are processed between pure-dots (name.value) and indexing (name["value"]). Not complete and indexing isn't yet available everywhere, but it should be easier to make it so.

This consolidation included error reporting. Now non-existent variables will always cause an error rather than being treated as false / "".

The other major format-breaking change is I removed for_loop variable which is a duplicate of forloop, the latter being shopify compliant.

This also saw a huge performance change for rendering

Before:

test bench_parse_template  ... bench:      24,825 ns/iter (+/- 1,831)
test bench_parse_text      ... bench:       3,702 ns/iter (+/- 255)
test bench_parse_variable  ... bench:       5,294 ns/iter (+/- 447)
test bench_render_template ... bench:      38,182 ns/iter (+/- 3,325)
test bench_render_text     ... bench:       8,858 ns/iter (+/- 541)
test bench_render_variable ... bench:      10,018 ns/iter (+/- 503)

After

test bench_parse_template  ... bench:      23,337 ns/iter (+/- 2,152)
test bench_parse_text      ... bench:       2,662 ns/iter (+/- 191)
test bench_parse_variable  ... bench:       4,544 ns/iter (+/- 306)
test bench_render_template ... bench:       8,996 ns/iter (+/- 482)
test bench_render_text     ... bench:       2,231 ns/iter (+/- 133)
test bench_render_variable ... bench:       7,188 ns/iter (+/- 412)