html/template automatically HTML-escapes special characters to prevent
XSS, which can silently corrupt arbitrary script data. Since the output
is binary script bytes and not HTML, this escaping provides no benefit
and might breaks correctness.
In this commit, we add a new function, `ScriptTemplate` to make the
process of making custom Bitcoin scripts a bit less verbose.
ScriptTemplate processes a script template with parameters and returns the
corresponding script bytes. This functions allows Bitcoin scripts to be
created using a DSL-like syntax, based on Go's templating system.
An example of a simple p2pkh template would be:
`OP_DUP OP_HASH160 0x14e8948c7afa71b6e6fad621256474b5959e0305 OP_EQUALVERIFY OP_CHECKSIG`
Strings that have the `0x` prefix are assumed to byte strings to be pushed
ontop of the stack. Integers can be passed as normal. If a value can't be
parsed as an integer, then it's assume that it's a byte slice without the 0x
prefix.
Normal go template operations can be used as well. The params argument
houses paramters to pass into the script, for example a local variable
storing a computed public key.
